Samstag, 17. August 2019

Comprehensive Critique of Thomistic Natural Law Ethics Using the Example of Sexual Acts

A foreword

The ethical theory of Thomistic natural law always(!) consists of three elements, whereby the weighting of these elements varies from Thomist to Thomist: naturalism, supernaturalism, and intuitionism.

There is no ethical natural law, for example, which is one hundred percent and exclusively naturalistic or solely supernaturalistic. But here lies the catch. A theory could consist of 70 percent naturalism, 25 percent intuitionism, and 5 percent supernaturalism. The problem could be now that the naturalistic 70 percent are dependent upon the supernaturalistic 5 percent (which could include, for example, the spiritual and sacramental institution of marriage or a spiritual anti-hedonistic attitude or the pursuit of spiritual holiness or a spiritual understanding of love), that is, if one does not believe in the 5 percent and considers them to be absurd, then the 70 percent may also break apart in their conclusiveness. 

Moreover, Thomistic natural law assumes a Catholic-Christian intuition. If one does not have this intuition, the whole enterprise of this particular natural law will not be very convincing.

In order to criticize natural law, one only needs to look up these three aspects - naturalism, intuitionism, and supernaturalism - in a good ethics handbook of analytic philosophy. There you will find what you are looking for and see which systematic problems are linked to them. 

In my critique, I am more concerned with a theory that focuses on the naturalism aspect. Such a predominantly naturalistic theory of natural law then amounts to the so-called perverted faculty argument that was prominently represented by Elizabeth Anscombe back then and is popularized today by Edward Feser. 

I will mainly deal with the topic of natural law in connection with sexuality. Sexuality is the main topic of natural lawyers, even if they claim the opposite. In all other areas of life, natural law can easily be replaced by a different ethics; only with regard to sexuality does natural law seem to have a kind of monopoly. In this context, the perverted faculty argument plays a major role: 
Natural law thinkers earlier in this century often relied on the “perverted faculty argument” to demonstrate the moral illicitness of various sexual practices, prominently including contraception. According to that argument, procreation is the natural function of semen, of the human genitalia, or more generally of the “reproductive faculty. (Weithman, Paul J.: Natural Law, Morality, and Sexual Complementarity. In: Sex, Preference, and Family - Essays on Law and Nature. Edited by David M. Estlund and Martha C. Nussbaum)
But it shouldn't go unmentioned:
The best recent defenders of a Thomistic natural law approach are attempting to move beyond it [the perverted faculty argument]. (Pickett, Brent L.: Natural Law and the Regulation of Sexuality. A Critique. In: Volume 8 Richmond Journal of Law and the Public Interest 39 (2004). https://scholarship.richmond.edu/cgi/viewcontent.cgi?article=1076&context=jolpi)
So there are at least two types of Thomist natural law scholars who must be somehow philosophical opponents. Here is a general characterization:
Some approaches to natural law have emphasized the physical structure of nature (e.g., the framing of the traditional Roman Catholic position on birth control). Such accounts often assume a teleology which articulates the ends of physical functions. Other approaches to “nature” focus more on the nature of practical reason rather than the teleology of natural physical structures (see, for example, Finnis, Grisez, and Boyle or Gomez-Lobo). (KEVIN WM. WILDES, S.J.: WHOSE NATURE? NATURAL LAW IN A PLURALISTIC WORLD)
It should be noted at this point that the physical structure of human nature is not to be interpreted only outwardly, for that would literally be superficial, crude and meaningless, but above all, it is to be understood in such a way that there is a felt inner side to it, that is, that there is something inner to it or a corresponding inner domain that is decisive. 

In brief, natural law means the following: To act contrary to nature is morally evil, to act according to nature is morally good. But "nature" is subject to many different descriptions. Any account of "nature" is a particular account that builds in certain views of moral reason and certain rankings of moral values.

Here is a summary and an overview of what I said previously (whereby it is not directly said here that Thomist natural law is a composition, which it undeniably is):
Natural law is more a broadly pluralistic tradition of doing ethics than a precisely formulated ethical theory. People in the natural-law tradition are much impressed by the work of St. Thomas Aquinas; but after that they go in different directions. For example, some in the natural-law tradition base ethics on God’s will, along the lines of supernaturalism. Others in the natural-law tradition base ethics on a naturalism that deduces ethical norms from empirical facts about desires— or, alternatively, on an intuitionism that appeals to moral intuitions or selfevident moral truths. But all are (or at least claim to be) true to the inspiration of St. Thomas Aquinas. Most in the natural-law tradition are nonconsequentialists, and defend exceptionless norms (for example, against killing the innocent). But a few adopt a “proportionalism” close to utilitarianism. Still others follow a virtue ethics. (Harry J. Gensler - Ethics: A Contemporary Introduction, Chapter 13)
The following can be said as the closing words of the preface:
The fact that there are numerous theories of natural law, some at great variance with one another, indicates that moral truth may not be as easily apprehended as proponents have asserted. (KEVIN WM. WILDES, S.J.: WHOSE NATURE? NATURAL LAW IN A PLURALISTIC WORLD)
I) My biggest problems with the classical Thomistic theory of natural law are the following:

What about the compatibility of natural law with the Christian doctrine of original sin or with any philosophical pessimism at all? After all, I don't want to act according to sin. If the whole of (human) nature is sinful, i.e. morally reprehensible in itself, how can it at all be a yardstick for truly moral action? In this regard, the voice of a biblical Christian can be quoted that could be representative of many Christians: 
For the Christian, however, nature is not the standard, because the world of nature is a fallen world, a world in rebellion against God and infected by sin and death. For a standard, we must look beyond nature to God. (http://darashpress.com/articles/natural-law-summary-and-critique)
Also, Aristotle says that "nature is demonic, but not divine" (De divin. 2, 463b 15). The prominent Thomistic natural law representative Edward Feser does not go into this problem (at least I didn't find anything in his works in this respect). How far does sin extend for him? The whole of (human) nature or just certain parts of it or everything in the world except the spirit? Secretly, Feser seems, as I suspect, to advocate a Pelagianism that completely denies an inherited original sin, the Augustinian one (Saint Augustine let all nature be corrupted by original sin)In any case, one thing is certain for me, ethics of natural law is only compatible with heretical Pelagianism (I know that the Catholics represent a kind of semi-Pelagianism, which in my view is only wishy-washy and commits the fallacy of the middle ground). I should have said more about original sin in connection with natural law, but the concept of original sin is such a vague and strange concept that theologians always produce it when they get into philosophical embarrassment. 

According to the sociobiologist Robert Trivers (The Folly of Fools: The Logic of Deceit and Self-Deception in Human Life), man has an innate natural ability to lie to himself (faculty for self-deception) also in order to make a lie more credible to others. Trivers can support this thesis well. Would the Church deny this natural (positive) existing and real trait of man, or would she attribute all so-called "evil" traits to sin? Unfortunately, I do not know. But what I do know: If the natural lawyer were to arbitrarily refer to the allegedly sinful part of man, an evasive maneuver would take place. 

Fritz Mauthner is absolutely right when he says the following:
All natural law scholars were and had to be: optimists, thus bad Christians; they believed in the goodness of human nature[.] (my own translation from German, Fritz Mauthner - Wörterbuch der Philosophie, dictionary of philosophy)
We will say more about this further below, but that much should already be mentioned: Natural law is based on philosophical optimism (man is good in his innermost being, life is worth living, children should be conceived, because natalism is according to nature and hence morally good) which cannot be justified any further. As a natural lawyer I have to look at nature and life optimistically and by no means pessimistically. But whether one is a pessimist or an optimist is rather a personal, subjective matter, not a logical one. So there is a weak basis for natural law, which only addresses optimists. Antinatalists and pessimists will fall by the wayside. Could Alexander von Humboldt be into natural law when he says this:
I regard marriage as a sin and propagation of children as a crime. (https://www.goodreads.com/quotes/9106761-i-regard-marriage-as-a-sin-and-propagation-of-children)
I will now quote two texts that illustrate why human nature is not necessarily good. But first, there is still something to be said about Aristotle beforehand, who has laid some foundations for natural law. Aristotle is far from developing a theory of subjectivity, but there is a place in his works where he comes a little closer to such a theory. There he talks about a person’s anger and transfers his famous form/matter distinction to it. The felt anger itself is the form and the bubbling of the blood is the matter of this human activity and state, whereby the anger cannot be limited in its expression only to "the bubbling of the blood" (it belongs more to it like hitting on a table) and there is no anger that is not directed at something or someone (even if it is directed inwards against oneself). If, however, every form always implied purposefulness (final causality) and every affection or aversion (inner inclination, disposition or propensity) were also a form, this would clearly have to be of natural law relevance. Thus Emmett Barcalow’s following remarks would be fully justified:
It may be that human beings, or at least male human beings, are naturally aggressive and prone to violence. After all, war and fighting seem to be such universal pastimes of men in all ages that one might conclude that the inherent nature of male human beings includes a strong tendency to behave violently. If that is so, should men act in accordance with their inherent nature or should they try to resist their inherent natural tendencies? Similarly, many people believe that the image of childhood as a time of innocence and purity is sentimental nonsense. In their view, children are inherently cruel and are brought to extinguish or control their inherent cruelty only through education and socialization. Consider the tendency of children to mercilessly taunt or bully those who are weaker than or different from themselves. We might maintain that the purpose of moral education is not to encourage people to give their inherent natures free rein but rather to tame their inherent natures. Similarly, suppose that human beings are inherently selfish or primarily selfinterested and that altruism is not in accordance with the inherent nature of human beings. If this were true, would it follow that altruism is immoral and contrary to reason because it is not in accordance with the inherent nature of human beings? (Emmett Barcalow - Moral Philosophy: Theories and Issues)
Alan Gewirth can also add something important: 
Is reason the only operation that is distinctively human and hence specifically natural to man? Consider the following: using an opposable thumb; being capable of lying, cheating, and stealing; following out the will to power; and, if we believe Freud, laboring under an Oedipus and Electra complex. Each of these modes of operation pertains only to human beings; and on at least some plausible theories of human nature, they pertain to all human beings. This is, of course, a very old point. Let me remind you of what Plato had Glaucon say in the second book of the Republic: That "by nature all [human] beings pursue as good their own selfaggrandizement [pleonexian]" so that for all men "by nature, to commit injustice is good." Hence, if man's good is identified with his nature in the sense of his distinctive mode of functioning, then we shall have to say that man's good consists in his self-aggrandizement or his pursuit of the will to power or his acting out the Oedipus or Electra complexes, and so forth. But these versions of man's good, and the accompanying precepts, are, of course, very different from those upheld by Aristotle and Thomas Aquinas. We therefore get results that are both indeterminate and morally unacceptable. (ALAN GEWIRTH – THE ONTOLOGICAL BASIS OF NATURAL LAW: A CRITIQUE AND AN ALTERNATIVE)
And here is the problem:
If we say (as Aquinas does) that all people whose natural inclinations are not "corrupted by vicious habits" and "darkened by passions and habits of sin" acknowledge [the] natural laws, we can ask, in turn, where do we get our criteria for deciding whose habits are vicious and sinful and whose are not? To rule out some natural inclinations as corrupt or sinful indicates that we are using a criterion in moral appraisal that is distinct from the natural law criterion of basing man's moral conceptions on his natural inclinations. What actually happens is that those moral beliefs that are incompatible with Catholic doctrine, and as a result are called corrupt and sinful, are simply arbitrarily labelled as "unnatural" and "abnormal." (Kai Nielsen - Atheism and Philosophy)
If the Thomist wanted to say that evil and bad can only be understood as the absence of good, then that is, apart from the fact that it must be justified philosophically, at first not credible at all. I could understand what is said by the Thomist in two ways. On the one hand, I could say, wonderful, there is no more evil, there would only be the neutral absence of good so that no more evil violations of natural law are possible. On the other hand, I could say that the expression "absence or lack of good" is merely a nominal difference from (present and positive existing) evil. "I do evil" would be identical to "I do the absence or lack of good". Not much is gained with that (except that it has caused conceptual confusion). Moreover, why are the Ten Commandments formulated with grammatical negation, when evil (like killing out of sheer homicidal lust or murderous desire) does not exist as a real thing on its own?

The next great difficulty in natural law is the well-known is-ought problem.

How is it possible to derive a moral ought from the mere fact that almost all human organs and abilities are functionally structured (one could perhaps also say teleologically)

From natural (necessarily proceeding) processes like fire or electrochemical reactions of the human body like digesting, even if Aristotelian ends are deeply anchored in these processes, I cannot derive any ought because we're only dealing with a pure happening and incident. 

The problem of is-ought in the biological realm can also be summed up in this way:
[T]o derive an exclusive moral prescription from an empirical observation of function [is] [...] to commit a category error. (Robin Gill - A Textbook of Christian Ethics) 
David Hume sought to show that facts about the world or human nature cannot be used to determine what ought to be done or not done. The derivation of command-like ought-statements (that are intended to be person-related) from mere (empirical) is-statements regarding non-personal nature (even if nature is teleologically structured in an unconscious way) is logically impossible. An unconscious striving principle in nature (like biological abilities and functions) would only be a pure is-fact. If nature were as Nietzsche describes it, the Thomist would definitely agree with me here (as an example, when the soul is thrown into a world of nothing but many wills to power), even if he said that Nietzsche's view on nature was actually impossible (but for the sake of argument, the Thomist may let that pass)

Without the additional presence of a "rational being facing me", there simply is no ought possible, let alone a moral one. A deliberate demand of another person is, in fact, the essential condition for the realization of an ought in my mind. It does not matter whether this person is an angel, a demon, a spirit, the higher self (if man has a Kantian dual nature), or simply another human being. 

So there is a possibility of deriving an ought, namely from the will of someone else (it is also clear that the human will is always conscious, but not always self-aware). And I think it’s the only possibility to get an ought. The Thomist would have to go along with this train of thought in order to still be credible for me. 

Three translated quotations shall illustrate my point:
A good rule of thumb for understanding the practical "ought" is: Where there's an "ought", there's a will of someone else. If a ought to do x, it implies that someone wants a to do x. [...] And the one who is told that he ought to do something can always ask back who it is who wants it from him. (my own translation from German, Peter Stemmer – Normativität)
And:
Ought expresses a necessity, which however is not given by objective (or as such seen) conditions, but always includes the will of a foreign entity (instance) (therefore: demand). The entity (instance [authority]) is mostly a person who does not have to be named in the sentence, but as such is always clear (in the above examples: physician, author, legislator). (my own translation from German, Buscha/Helbig - Deutsche Grammatik, german grammar book)
Finally:
And it is obvious that such an “ought” is therefore nothing but the synthetic result of the encounter of subjects, each of whom, as a conscious end in itself, is a conscious willing in relation to the other, and thus also a conscious demanding. (my own translation from German, Gerold Prauss - Moral und Recht im Staat nach Kant und Hegel, Morality and Law in the State after Kant and Hegel)
How does this logic of action and speech look more precise? The first step would be something like this: A person, for example, demands help from me by turning to me and saying, "Help me, please" which means "I want you to help me". Of course, I immediately understand that I should help her. So we have gone from a will/demand of another person to a should or ought which is inside of me. I have to translate a will of another as an ought to for myself. 

God might come into question if he is understood as a person. More, God would have to be understood as a real person in a non-analogous sense, albeit as a supernatural higher super-person. You can’t get past it: No other person, no will of hers → no ought for me. We do indeed need a personal God as a morality demanding entity in natural law.

A scholastic sees it similarly. Francisco Suárez (1548—1617) would argue
that for a law to be genuine law and not just law in name it must be grounded in the legislative act of a superior. The obligatory force of natural law comes from God’s will. Contra Gregory of Rimini, that obligation would not be present had God not legislated or not existed at all.  https://iep.utm.edu/suarez/#SH3i)
Thus, without Divine Command (The Arbitrary Will of God) that demands something from me, there might be no true moral imperatives. Since God is apparently omniscient, omnipotent, and omnibenevolent, His Will would be binding (The will of society has otherwise historically always been considered binding.). 

Therefore, in order to function at all, a very sophisticated divine command theory would have to be incorporated into natural law so that God can guide us to good action.
So a Divine Command theory is hidden in natural law, because natural law is, after all, a part or aspect of the divine law given by revelation. God would somehow have to communicate with us through the purposefulness (final causality) of our physical and mental nature. But why should God communicate with me through my individual parts (bodily functions, organs, faculties) as the Thomistic theory seems to suggest and not through my preceding whole being? Why not through my conscience, like it has always been understood in Christian traditions? Parts whatsoever, my conscience gives me real clues or my true peace of heart gives me them. 

If I do not believe in God, this indirect "communication" fails, however, because Hume’s is-ought guillotine immediately takes effect. So it would have been a bad communication from the beginning, which is not worthy of God. God cannot expect an unbeliever to commit a fallacy. A quote from Kai Nielsen goes well with this:
Those who think they can discover what they ought to do from a discovery about “ultimate nature of reality” are tacitly assuming that what is metaphysically real or “ultimately real” ought to be done or ought to be. But it is not self-contradictory or even logically odd to say, “X is an ultimate reality whose nonexistence is inconceivable, but X ought not to be.” What is ultimately real could be evil or it could be quite neutral. There is no rule of language which indicates an identity of meaning between what is real and what is good or obligatory. To make such a rule by linguistic fiat and to claim that such a linguistic fiat ought to be accepted because it gives us a clearer, more adequate foundation for morals is itself an expression of a value judgment; and if such a value judgment is made, we must for the sake of clarity give up the idea that we can derive a moral statement from purely metaphysical statements and/or metaphysical and empirical statements alone. (Nielsen, Kai: An Examination of the Thomistic Theory of Natural Moral Law. In: Kai Nielsen: God and the Grounding of Morality 1991 https://scholarship.law.nd.edu/cgi/viewcontent.cgi?article=1038&context=nd_naturallaw_forum)
If I am agnostic to the existence of God, I must also state that this indirect communication is somewhat strange and bad too. There should, therefore, be for atheists and agnostics a direct revelation that tells them briefly to obey natural law. Since this would no longer do justice to a philosophical claim, it would also be out of the question. 

However, if I accept a God, then on the part of this God many important explanations and additional pieces of information are missing, like what exactly should I pay attention to, what could the disregard for consequences have for me, how exactly should one use this and that, etc., what is particularly important and what not and so on and so forth. So a huge divine manual would have to be given, which is obviously not the case. If I, as an absolute stranger to the world, see a traffic light and sign system, I can only assume an ought behind it when there are people who indirectly demand something with it, namely compliance with traffic law. If these people do not exist anymore, then one only has to deal with an as-is state, from which nothing ought worthy can result. And besides, I could not even derive the pure traffic rules from purely empirical observations of busy traffic (at least it would take me ages, and I still wouldn't have any certainty afterward). Without existing traffic (just pure emptiness on the streets, but with traffic lights working), I would be dependent on pure interpretations and speculations. There must be someone who explains the rules to me (preferably in my mother tongue) completely from the outset. In natural law, this someone is as already said missing. Not only that: Because of the alleged original sin, it is even more difficult to gain an overview.

If we accept our nature as a moral compass, we would be dependent on the complete arbitrariness of God with regard to this compass anyway. For he could have created another nature at will, thus another morality, and could still do so by miracles. If God orders nature teleologically, it remains unclear whether this order corresponds to his will at all, (on) how we should behave (strictly speaking, instead of God's will, one would have to talk about his wish, since his will could never go wrong and fail). His teleological order of the world, including the human body, perhaps only enables us the stage (with trustworthy regularities) on which we should then act according to a completely different, perhaps Kantian, ethics. But even if it would correspond to his will in the sense mentioned (as to how we should behave), it remains questionable whether his demands as a nonparticipant in every day (world-)affairs are really morally binding on us, questionable whether they, as impious as it may sound, are at all morally good (besides, the world may have been created by an evil being as in early Christianity the Marcionites thought or partly the Christian Gnostics; or the world may have been created by a demiurge as Plato assumed). Maybe the teleological order is just a (non-binding) recommendation from God on how to live a happy life, but nothing more (no legal morality, no penalties in an afterlife, and also no transfer of an unhappy life in this world into the hereafter). Grisez, an arch-conservative in moral matters, also sees problems:
Yet, it is not evident that God requires that this design [of all human nature] always be respected. (Germain Grisez: Contraception and the New Natural Law – Chapter 2: INADEQUATE ARGUMENTS https://catholicebooks.wordpress.com/2014/07/25/online-text-contraception-and-the-natural-law-by-germain-grisez/)
What about the nature of other beings? When I see a beetle or a turtle lying on its back and it cannot stand up by itself anymore, the effort by it to turn around expresses a goal orientation that I can only interpret as God’s demand. Would there then be a moral ought for me to intervene if I were present, especially if some person did this cruel deed on purpose? Or if a fly annoys me and it thereby only pursues its teleological nature (which must also be an expression of the divine will) and I kill the fly, as one of the gigantically many expressed wills of God, should no one really care? Why not? If the Thomist would say that we are not dealing here with an intellectual soul, that would be in any case unsatisfactory for me. It would only be a word thrown into the discussion, but not a substantial explanation. The natural law scholar must make very good arguments and not merely assertions that only the teleology of one's human body is morally meaningful. 

The universe (including all humans) is striving for entropy death and all material is slowly but surely swallowed up by black holes, all this I could interpret as purposeful, and thus as God's will, and finally as God's demand ("Entropy — the tendency for change to reduce overall order — is a teleological process. It is perhaps the overarching teleological process observable in nature." 
https://evolutionnews.org/2017/08/does-nature-show-purpose-reply-to-a-materialist-philosopher/). That is, we ought to help the universe (and indirectly ourselves) die, meaning that we ought to increase entropy (in our own bodies, in our lives and in the cosmos)? Aristotle says that a solid like a stone has a natural place, and this is the earth: 
To be there belongs to the essence of the stone. Therefore, the stone wants to remain on the earth - its natural place of residence. That is why you have to use strength to lift it up and even to hurl it up. But the stone retains its natural tendency to be on earth; it is responsible for its falling down again. This natural tendency prevails against the perverting force that has thrown the stone upwards against its nature, and so the stone, following its natural aspiration, returns to earth as its natural place. (Welsch, Wolfgang. Der Philosoph: Die Gedankenwelt des Aristoteles; translated from German by myself, The Philosopher: The world of thoughts of Aristotle)
Would it then be immoral to throw a stone into the air if Aristotle was right in his theory of natural places?

The is/ought debate is actually a sham debate on the part of the Thomists. They are not really interested in this problem. Because in their world I can move away from God or move towards Him (it's like a highly cynical, macabre, and darkly twisted mechanism). God does not force me to do anything, nor does he demand anything. I only have to take the consequences: heaven or hell. So we are dealing solely with metaphysical consequentialism. One could also say that God is interested in the highest number of souls in heaven and thus in the lowest number in hell: so the greatest possible happiness of the greatest possible number (theological utilitarianism). But these things are all a matter of pure faith.

Here are some helpful quotes on the subject:
Natural moral law theorists confuse talking about what is the case with talking about what ought to be the case. They confuse dejure statements with de facto statements. A statement about what people or what normal people seek, strive for or desire is a factual, non-normative statement. From this statement or from any conjunction of such statements alone no normative (de jure) conclusions can be validly deduced except in such trivial cases as from "He wears black shoes" one can deduce "He wears black shoes or he ought to be a priest." But this simply follows from the conventions governing the disjunction "or." Moreover, because it is a disjunction it is not actually actionguiding; it is not actually normative. To discover what our natural inclinations are is simply to discover a fact about ourselves; to discover what purposes we have is simply to discover another fact about ourselves, but that we ought to have these inclinations or purposes or that it is desirable that we have them does not follow from statements asserting that people have such and such inclinations or purposes. These statements can very well be true but no moral or normative conclusions follow from them. (Kai Nielsen - Atheism and Philosophy)

Even if one could exhaustively describe the elements of our nature, the claim that we are morally obliged to act in accord with them, or to prefer “natural” uses to “unnatural,” could be made only as something additional and adventitious to the whole ensemble of facts that this description would comprise. Otherwise we could not see it as a moral good at all, but only as a negotiable feature of private taste. The assumption that the natural and moral orders are connected to one another in any but a purely pragmatic way must be of its nature antecedent to our experience of the world. I know of many stout defenders of natural law who are quick to dismiss Hume’s argument, but who — when pressed to explain themselves — can do no better than to resort to a purely conditional argument: if one is (for instance) to live a fully human life, one must then . . . (etc.). But, in supplementing a dubious “is” with a negotiable “if,” one certainly cannot arrive at a morally categorical “ought.” (David Bentley Hart - Is, Ought, and Nature’s Laws. In: A Splendid Wickedness  and Other Essays)

The most gallant of Feser’s non sequiturs is his claim that, because reason necessarily seeks the good, there exists no gap into which any Humean distinction between facts and values can insinuate itself. But obviously the gap lies in the dynamic interval between (in the terms of Maximus the Confessor) the “natural” and “gnomic” wills. The venerable principle that the natural will is a pure ecstasy toward the good means that, at the level of gnomic deliberation, whatever we will we desire as the good, but not that philosophical theory can by itself prove which facts imply which values, or that the good must naturally be understood as an incumbent “ought” rather than a compelling “I want.” Feser asserts that “purely philosophical arguments” can establish “objective true moral conclusions.” And yet, curiously enough, they never, ever have. That is a bedtime story told to conjure away the night’s goblins, like the Leibnizian fable of the best possible world or the philosophe’s fairy tale about the plain dictates of reason. (David Bentley Hart - Nature Loves to Hide. In: A Splendid Wickedness  and Other Essays)

Feser claims that he does not have an “is-ought” problem. Maybe so, but he has merely replaced it with a “natures’s End - ought” problem that is equally tenacious. He still faces the question of “So nature intends that I X. But why should I X?” Seems to me he has gained nothing from that move. (Some internet commentator)
When the body and the mind constitute together a genuine hylomorphic unity, so that in the end, physical processes have an influence on the mind, and vice versa, the question arises as to how one can be sure of the correct insight into natural law.

If someone is suffering from a mental disorder or physical illness and thinks he knows exactly what natural law is supposed to look like, how much faith can one give that person? Diseases and bodily impairments can affect both judgment and cognition. A person who is completely blind from birth on, for example, has her very own access to things and their qualities (one gets the impression that naturalistic natural lawyers, who concentrate only on the external physical structure of nature, are very much dependent on the sense of sight when establishing and justifying their ethics). A heavily manic-depressive person (and such a person may well be religiously motivated) should not be too sure about ethics, especially natural law ethics. As is well known, left- and right-wing people constantly give the opposite side the diagnosis of a pathological condition. In this way, the respective notions of what human nature is about are to be devalued.

Cultural influences, deeply rooted customs, and traditions could also be further obstacles to grasping the true teleology of man in a clear and unbiased way. One example would be circumcision. Certain religions fully endorse this practice, although it is strictly speaking mutilation of the genitals, contrary to their teleology. That circumcision is an ethical offense would never be understood by these religions. A tradition thousands of years old has, so to speak, blinded them in this respect. Perhaps the Western world is also partially blinded by cultural Patriarchy? Or later by Postmodernism? Perhaps all people who have no access to Thomistic natural law, including the extremely moral Kantians, are simply groping in the dark. Or the natural lawyers are the ones who darken the true morality? Who (really) knows? Perhaps our reproductive organs are nothing more than a means of proving oneself morally by resisting devilish sexual seduction. Maybe our Western culture has only obscured this goal? Who can tell? It seems so easy to conceal natural law with a wafer-thin cultural veil, which does not speak in favor of this theory.

Does my inability to recognize the divine order of nature then relieve me of all moral responsibility? If I "in false knowledge" violate the supposed natural law, am I then accountable? These questions are not irrelevant since natural law is based on very specific insights. Be that as it may, the recognition of a true purpose, even from a physically and mentally healthy state and from a scientifically unbiased point of view, was not an easy task even for Aristotle. For
the more matter is involved in the formation of an object [like an organ], the more unclear the purpose can be for the scientist. (my own translation from German, the Catholic Robert Spaemann on Aristotle´s philosophy in Robert Spaemann, Reinhard Löw - Natürliche Ziele, natural purposes)
Alan Gewirth can add:
In the Posterior Analytics, the Topics, and especially the first book of the Parts of Animals, Aristotle pointed out the very great difficulties that confront philosophers or scientists who try to ascertain the characteristics that comprise the essential nature of any species. He showed that there is no simple way of equating this essential nature either with the ultimate matter, or with the sensible form, or with the generic functioning, or even with the specific functioning of each species. (GEWIRTH, ALAN - THE ONTOLOGICAL BASIS OF NATURAL LAW A Critique)
Then, nevertheless, the following question arises: Do people first have to ask zoological and anatomical experts before taking action to find out what human nature might contain (because if anyone knows anything at all in that regard, it will be only the sober scientist)? 

In fact, the Church has often had to be instructed by experts in human anatomy. The position during sex, for example, in which the woman is at the top, was condemned by the church for a long time because it allegedly hindered the goal of reproduction. According to Church doctrine, the semen would have come out of the woman’s body again. But newer physiological and anatomical findings did not confirm this, and the frowned-upon sex position then had to be permitted meekly by the Church. This is a prime example of a great weakness that lies in the ethics of natural law, namely that it depends on scientific knowledge, with the consequence that its teachings can always be falsified. So one would have to give to the following conclusion:
The facts are revisable in ethics, then, for exactly the same reason that they are revisable in other disciplines — because our knowledge of ourselves and of the world is constantly evolving. Therefore, our conclusions in ethics will be provisional at best. (Murray, T. M. - Thinking Straight About Being Gay)
Grisez describes another example of a discrepancy between a premature acceptance of the church and the later correction of scientists:
The older scholastics who thought that feminine “semination” is analogous to male ejaculation were mistaken in their physiological facts. If they had not made a mistake on this point, many of them would have been unable to show the immorality of female masturbation because it has no objective significance for the reproductive function. (Germain Grisez: Contraception and the New Natural Law – Chapter 2: INADEQUATE ARGUMENTS https://catholicebooks.wordpress.com/2014/07/25/online-text-contraception-and-the-natural-law-by-germain-grisez/)
And the subordination of women
to men was long regarded as an indisputable moral [because indeed biological] certainty – and of course as an immutable part of natural law. Saint Thomas, for example, was very, very sure about this. (my own translation from German, Andreas Edmüller - Die Legende von der christlichen Moral, The legend of Christian morality)
The gay Thomist David Berger who was suspended by the Catholic Church summarizes the whole thing:
Even the most faithful Thomas pupils today will no longer want to claim, as Thomas does, that women are merely failed men who arise when the weather is warm and humid at the time of conception, because this climate weakens the sperm of men. The fact that the Catholic Church nevertheless clings desperately to the resulting conclusions on some points, especially those concerning sexuality and the role of women in the Church, is another story. Especially in the field of anthropology and thus also of moral theology it would be possible to compile a whole book with similar theses of Thomas Aquinas, which today only sound abstruse to us, but at that time reflected the latest state of science. Thomas’ ideas on homosexuality – and on the death penalty as an appropriate answer to it – will also have to be classified here, under the subordinate temporal condition. Against this background it becomes clear that one can be a Thomist and gay at the same time. (my own translation from German, Berger, David - Der heilige Schein: Als schwuler Theologe in der katholischen Kirche, The Holy Pretense: As a Gay Theologian in the Catholic Church)
The main objection to natural law morality is then that even if there were absolute natural law standards we could never know if we had found them. That's why it wouldn't be any of our business. In Epicurian terms, God is not morally interested in us (otherwise he would give us more clarity about natural law), so we are not morally interested in his alleged natural law either. 

Thomist Anthony J. Lisska, for example, even admits that 
Aquinas was almost agnostic about [the] possibility of knowing essences… (quoted from: Shalina Stilley: Natural Law Theory and the "Is"--"Ought" Problem - A Critique of Four Solutions https://epublications.marquette.edu/cgi/viewcontent.cgi?article=1059&context=dissertations_mu)
Lisska cites the following passages from Aquinas:
The essential principles of things are unknown to us. (De Anima I lec. 13) ―Essential differences are unknown to us.‖ (De Veritate, q. 4 a. I ad. 8) ― We are ignorant of many of the properties of sensible things, and in many cases we are unable to discover the proper nature even of those properties that we perceive by the senses.‖ (Summa contra Gentiles, I, 3) (quoted from: Shalina Stilley: Natural Law Theory and the "Is"--"Ought" Problem - A Critique of Four Solutions https://epublications.marquette.edu/cgi/viewcontent.cgi?article=1059&context=dissertations_mu)
Substantial forms are unknown to us (De veritate IV, I ad 8; cf x, I, & ad 6).

We do not know even the essence of a fly. (In Symbolo apostolorum I)
One does not even comprehend the nature of a fly. How, then, can one fully understand the nature of the complex human sexual faculty, given, in addition, that the telos of each faculty can change according to context?

On the subject of the recognizability of natural law, Jacques Maritain, an important philosopher of Thomistic existential philosophy, says that one simply knows deep within oneself (in a mystical intuition) what natural law is. But to present such a statement makes the whole thing more than questionable. In (vague) mystical waters, we clear thinkers have nothing to do. Kai Nielsen describes his problem with Maritain:
As Maritain makes perfectly clear, it is a direct, immediate, nonconceptual "knowing through inclination" or "knowledge through connaturality." And it is important to remember here that Maritain is not just trying to urge a view of his own but is also trying to elucidate what Aquinas really meant when he said we have knowledge of the natural moral law. 
[...] 
Let us look a bit more carefully at this doctrine, for it is, according to Maritain, central to the Thomistic theory of natural moral law. Unfortunately, as Maritain also emphasizes, it is an extremely obscure doctrine. This so-called "knowledge through inclination" seems unlike what we ordinarily call "knowledge." It seems more like a feeling or an attitude. I am at a loss here to find an intelligible use for "knowledge," and I am far from sure that we can see clearly from Maritain's account exactly what it is that he wishes to say.
[...] 
To call something that cannot be conceptualized or expressed "knowledge" seems like an early retreat into a kind of obscurity that makes philosophical appraisal impossible. 
[...] 
"Natural moral laws are self-evident in themselves" and "Natural moral laws are known through inclination" are two exceedingly obscure statements that raise more problems than they solve. The doctrines they express hardly function as aids to a rational solution of the problems connected with our knowledge of good and evil.
I must also completely agree with Robert Anton Wilson when he says the following:
Similarly, the Natural Law theorist (or any other metaphysician) tells you about abstractions with capital letters, and he talks about these marvelously transcendental entities, and he talks, and he talks…and if the hypnosis works, the abstractions suddenly seem as “real” as, or even more “real” than, a ham sandwich and a cup of coffee. (Robert Anton Wilson - Natural Law, or Don’t Put a Rubber on Your Willy)
And:
Thus, Natural Law seems like a spook in Stirner’s sense, a disguised metaphysics in which people can claim they are not rationalizing personal prejudice or doing what they want but are only dummies through which the Great God Natural Law is speaking and acting. (Robert Anton Wilson - Natural Law, or Don’t Put a Rubber on Your Willy)
With all the talk of a natural law intuition, only a Catholic intuition is actually to be created. Nothing more.

Nevertheless, scientific research cannot ignore an encounter with philosophical epistemology. And so, in order to theoretically avoid all possible epistemological problems, Aristotle still had to adopt a strict and highly speculative body-mind dualism (a ghost in a "living" organism instead of in a Cartesian machine), i.e. he had to go beyond his self-developed Hylemorphism so that a reliable perceptibility of the essence of things is guaranteed, at least in principle, which calmed all dull epistemology loving people. But this has brought great darkness into his "Soul Theory". The mind
has a special position, it does not fit seamlessly into the concept of the body-soul-unity. This perhaps explains why Aristotle's conception of the soul - his dissolution of the body and soul problem - did not become as effective historically as one might have expected. Aristotle himself encouraged this failure by not being able to integrate the dimension of the mind into his concept - and it was precisely the question of the mind that led to the dualism of two substances in modern times. (my own translation from German, Wolfgang Welsch - Aristoteles)
J. L. Ackrill also stresses the problem:
There is, however, as we have seen, one very grave difficulty in his overall theory of which he is aware, not the problem of private experience but the problem of pure thought. Supposing that pure thought requires no physical organ or physiological correlate, Aristotle must hold that this kind of soul, nous, can exist in separation from the body; but he finds it difficult to say much that is clear and useful about this activity without an agent, this form without matter. (J. L. Ackrill - Aristotle the Philosopher)
Is natural law compatible with biological evolution? This is the most important question in times of prevailing secularism and established theories of the evolution of quite a lot of things. But to begin with, I will give a brief overview of what evolutionary theory means for humans in general. First and foremost, the structure of human nature is as it is because certain morally neutral adaptations of certain "selfish" genes were simply more successful than others in the distant past. Then it can be said that evolution is not simply a thing of the past, but continues tirelessly through sexual selection, which also takes place beyond good and evil. Some people could suddenly get a third arm through mutation, which may not initially be as fully functional as our two known arms, as it is more like a stump with very rough fingers. But it could be very sexually attractive to the opposite sex. Through sexual selection, more and more people would have three arms, which would also become more and more functional because the better ones would be even more attractive. The game can now be played with all organs, tendencies, and behaviors.

The major result is that we can no longer assume that sharply definable animal species exist. This is because we can no longer say exactly where the contours of such an alleged species begin and where they end. And that is not due to a lack of understanding on our part but to the nature of life itself. In other words, there are no species in the Aristotelian sense. Of course, man can decide artificially and arbitrarily when a species should be a certain species after he has invented one. But the philosophical problem is not solved by this. Nature simply does not make the complete scope of a presumed species conceptually accessible to us because this scope does not exist in (or as) biological reality. 

Even if one were to cling maniacally to Aristotelian species, their exploration would always turn out to be insufficient and inadequate for us. We can never say whether new emerging properties represent an abnormality of a species or whether they represent a new species. All standards for the assessment of Aristotelian species would be philosophically doomed to failure in their credibility since they are based only on arbitrary fixations or tacit value criteria. 

Which (essential) characteristics belong to a certain living being and which do not can no longer be answered for sure if one takes a close look at its evolutionary history. Every living being as a substantial form has not simply popped up as such out of nowhere. It just grew out of the continuous phylogenetic tree of life and found its place there, but only in a gradual (blood-)relation to its predecessors. As Dawkins once said, there was no first Adam and therefore there will be no last one. It is no longer possible to point the finger at the place where man supposedly became man.

Neither Aristotle nor Aquinas believed in the reality of the evolution of species:
Aquinas believed that all human beings have a fixed, uniform human nature – this led him to maintain that there was a fixed natural law [...] for human beings. (Vardy, Peter - The Puzzle of Ethics)
Not only was Aristotle not an evolutionist, but he was also even against any kind of evolution. Biologists, including Darwin, always had great admiration for Aristotle, but they also had to admit with regret that they could not count him among the evolutionists. 

After all, Darwin had initiated the end of the metaphysical species. In truth, Lamarck with his principle of inheritance of acquired characteristics first rejected the essentialist species concept with good reasons. Lamarck replaced the static world view with a dynamic one in which not only the species but the entire ladder of living beings and the entire balance of nature are constantly in flux. There is no doubt that he deserves credit for being the first to advocate a consistent theory of genuine evolutionary change.

According to Schopenhauer, Lamarck must have somehow assumed the existence of a primordial being that is completely unspecific, but which is essentially life as such and possesses enormous, though very slow-paced, plasticity. Something like the thing or the blob from the horror movies. With the exception of the characteristics drive for life and plasticity or mushiness or whatever, all characteristics up to the human being on the evolutionary path are accidental characteristics, like the fact that the human being has two legs. If one day it should turn out that Lamarck was not completely wrong after all, that is, that the correctness of his principles is scientifically proven, Thomism has another insurmountable problem (besides Darwinism, which does not exclude Lamarckism)

So the only imaginable "fixed" and essential form would be life as such, unspecific, and considered in itself. But not to forget: The essence of life, nevertheless, is change (even if the change is very small, insignificant, and invisible to the human eye). The energetic intensity of life could always change (at least after every generation). Life essentially means change, profound modification, and transformation.

A true theory of evolution must postulate a gradual transformation of one species into another ad infinitum. Accordingly, organisms change over time. All this change is slow and steady and stately. If there is enough time, small changes within a species can accumulate into large changes that create new species.

Here Dawkins also gives an overview of evolution:
Descendants can depart indefinitely from the ancestral form, and each departure becomes a potential ancestor to future variants. Indeed, Alfred Russel Wallace, independent co-discoverer with Darwin of evolution by natural selection, actually called his paper 'On the tendency of varieties to depart indefinitely from the original type'. If there is a 'standard rabbit', the accolade denotes no more than the centre of a bell-shaped distribution of real, scurrying, leaping, variable bunnies. And the distribution shifts with time. As generations go by, there may gradually come a point, not clearly defined, when the norm of what we call rabbits will have departed so far as to deserve a different name. There is no permanent rabbitiness, no essence of rabbit hanging in the sky, just populations of furry, long-eared, coprophagous, whisker-twitching individuals, showing a statistical distribution of variation in size, shape, colour and proclivities. What used to be the longer-eared end of the old distribution may find itself the centre of a new distribution later in geological time. Given a sufficiently large number of generations, there may be no overlap between ancestral and descendant distributions: the longest ears among the ancestors may be shorter than the shortest ears among the descendants. All is fluid, as another Greek philosopher, Heraclitus, said; nothing fixed. After a hundred million years it may be hard to believe that the descendant animals ever had rabbits for ancestors. Yet in no generation during the evolutionary process was the predominant type in the population far from the modal type in the previous generation or the following generation. This way of thinking is what Mayr called population thinking. Population thinking, for him, was the antithesis of essentialism. (RICHARD DAWKINS - THE GREATEST SHOW ON EARTH)
And Ernst Mayr himself, a pioneer, and distributor of population thinking, should also be quoted: 
What is population thinking and how does it differ from essentialism? Population thinkers stress the uniqueness of everything in the organic world. What is important for them is the individual, not the type. They emphasize that every individual in sexually reproducing species is uniquely different from all others, with much individuality even existing in uniparentally reproducing ones. There is no "typical" individual, and mean values are abstractions. Much of what in the past has been designated in biology as "classes" are populations consisting of unique individuals.

[...]

This uniqueness of biological individuals means that we must approach groups of biological entities in a very different spirit from the way we deal with groups of identical inorganic entities. This is the basic meaning of population thinking. The differences between biological individuals are real, while the mean values which we may calculate in the comparison of groups of individuals (species, for example) are manmade inferences.

[…]

Francis Galton was perhaps the first to realize fully that the mean value of variable biological populations is a construct. (Ernst Mayr - The Growth of Biological Thought Diversity, Evolution, and Inheritance)

The homosexual acts abhorred by most Thomists consequently belong to the human population just as much as the heterosexual ones. Among homosexuals, only the objects of their erotic-sensual inclinations have shifted and the functions of their genitals have changed or rather expanded. From the point of view of population thinking, all this is still something entirely natural.

The critique on essentialism does not merely deal with the naming of species which may only be concepts of the human mind without actual existence or biological reality, which were invented to refer to a large number of individuals collectively. Rather, it is a criticism of so-called biological essentialism, according to which every species is characterized by its unchangeable nature (eidos) and is separated from all other species by a sharp discontinuity. According to this view, the individuals of a species are not in a special relationship to each other; they are merely expressions of the same eidos. The variation is the result of an imperfect manifestation of the eidos. Representatives of the Christian faith in creation and Aristotelians are biological essentialists. That is simply so. For them, there are everywhere exactly delimited species, species that remain the same and do not change. But their thesis has now become untenable. 

So one has to ask oneself: How can nature in its organic make-up provide an absolutely firm and credible yardstick for moral action if it is proven to be subject to a constant evolutionary process that is mercilessly brutal. For some, nature is even subject to successive change in almost all its facets. But you don’t need to go that far to question natural law. The survival and reproductive ability of an organism alone, as well as the corresponding specific characteristics that have developed in the course of natural selection and can still change from generation to generation (already existing organs could either get additional functions in the course of evolution or they could simply atrophy to complete inactivity), are unlikely to be helpful in making moral decisions. Unless you are a follower of social Darwinism. Apart from that, if evolution is to be seen as a guide to finding out how people should behave, we must first ask what end evolution is heading to. But any goal would be based on pure speculation, and it looks more likely that there is no goal. In addition, natural law ethics would have to follow the course of human evolution (which may be as slow as it is assumed, but in theory, it doesn’t change anything), i.e. in principle, it would have to adapt gradually to this course in order to remain up to date with the latest ethical standards (it remains an open question of whether it should stick to the average person or to the exceptional one). However, this is not compatible with the idea of a uniquely true, eternal, objective morality. The Book of Nature is not "written down" once and for all, it rewrites itself here and there. 

After all, man has evolved evolutionarily and this development has certainly not covered itself morally with glory, so that it is no longer possible to speak of a metaphysical human species, but only of a biological one in a loose sense. It seems strange to me to assume that God, according to Thomism, unites the intellectual soul with the fertilized egg and its genetic material that it is supposed to be a moral standard, considering that this material has a very bloody developmental history.

In the world of evolution there are only individual living beings ("egoistic" genes with their survival machines), between all of which only (blood) degrees of kinship (in the phylogenetic tree or family tree of life) exist, whereby we simply lose the uniform Aristotelian form of man as a supertemporal ethical model and swap, for example, a Thomistic value scale for an evolutionary one. Thomistically good was something that most closely matches its fixed and generalized form. Evolutionary good or "fit" is something that is as far as possible adapted to its environment and has enormous reproductive advantages. These are two very different goods. When the environment changes, the meaning of good or "fit" also changes. That which is considered good or "fit" in the 21st century is no longer the same as it was fifty thousand years ago. If you consider that there are glasses, myopia is much less of a problem today than it was back then.

The genetic relationship of all living beings, including humans, shows that one can at most assume one life form (DNA life form) that can have many different gradual manifestations. If we had only compared all the selfish genes that built their survival machines, we would never have come across completely isolated animal species. 

Michael Hauskeller sums up everything said with his words:
In contrast to Aristotle we know today – or think we know – that species are not natural kinds. Rather, the notion of species is a convenient classification, and not a biological reality. It helps us to bring some order into the world of living organisms, but the “real world consists only of individuals who are more or less closely related to each other by virtue of descent from one or more common ancestors”. Species change and evolve, with the effect that neither diachronically nor synchronically can we always determine to which species a certain individual belongs. Nor is reproduction only possible between individuals which are said to belong to the same species. (MICHAEL HAUSKELLER - Telos: The Revival of an Aristotelian Concept in Present Day Ethics. Published in final edited form as: Inquiry (Oslo). 2005 Feb; 48(1): 62–75. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1351104/)
The (platonic) idea of man is dropped by the assumption of evolution. We then have only human individuals. Homosexuals can no longer be compared to a timeless form or universal of man, or rather, an Aristotelian form that lasts forever, and that supposedly doesn't include homosexuality. The homosexual simply does what corresponds to his nature and fails evolutionarily. However, this would only be a neutral and non-judgmental observation, although the verb failure is preloaded in terms of human value. We can even use an interpretation of Aristotle to include homosexuals without prejudice, and say this:
At any rate, the good Aristotle talks about is not the good of the species, but the good of an individual. (MICHAEL HAUSKELLER - Telos: The Revival of an Aristotelian Concept in Present Day Ethics. Published in final edited form as: Inquiry (Oslo). 2005 Feb; 48(1): 62–75. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1351104/)
And even so, homosexuality could be natural:
It could be (and there is no evidence for this) that in the face of an overcrowded world, nature produces an increase in those genes which direct sexual activity away from procreation. (Vardy, Peter. The Puzzle of Ethics)
Here are some other theories:
An Italian research team found that the evolutionary origin and maintenance of male homosexuality in human populations could be explained by a model based around the idea of sexually antagonistic selection, in which genetic factors spread in the population by giving a reproductive advantage to one sex while disadvantaging the other. https://www.sciencedaily.com/releases/2008/06/080617204459.htm
And:
One possible explanation is what evolutionary psychologists call the "kin selection hypothesis." What that means is that homosexuality may convey an indirect benefit by enhancing the survival prospects of close relatives. https://www.sciencedaily.com/releases/2010/02/100204144551.htm
If nevertheless, one persistently wants to hold on to supertemporal, fixed and generalized forms of living beings, one must admit that the chain of evolutionary (gradual) changes (from bacteria to man) represents a chain of deformations and deviations (which, according to Aristotle, are to be regarded as bad in terms of value) from the "very first" (originally good) life form. Most of the evolutionary chains broke off prematurely, and some, such as those that reached man were by chance and with much luck successful. The idea of evolutionary adaptation to external circumstances is completely absent from Aristotelian thought. It is also important to bear in mind that evolution does not only affect the physical appearance of a living being, but also its behavior and internal dispositions.

Just because an organism is or has been successful in evolutionary terms does not make it a supertemporal and permanent form of life (Aristotelian species). And what is quickly perishing in nature cannot simply be described as a pathological deviation from an original life form. For external circumstances that arise by chance (and sometimes change quickly, sometimes slowly) and adaptability play a decisive role here. The "last" ones of a population, which are somewhat different in their nature, could then quickly become the "first" and, after some time, the only ones in their population. Anyone who wants to understand an organism in its development and in its specific life expressions must understand the environment in which it lives. If we negate the formative elements of chance and the environment, we deprive ourselves of a decisive means of understanding the emergence of new life.

If Thomists accept evolution, then hardly the version of Dawkins, namely that of gradualism. They should rather accept Gould’s version. But nevertheless, according to Aristotelian principles, no "eternal" species can give birth to a different "eternal" one. That is an Aristotelian impossibility. Therefore, the Thomists might have to be hidden creationists who see divine creations in the course of (a pseudo) "evolution".

Here a text passage shall be quoted in order to clear up all possible misunderstandings about Gould: 
Hence Gould and Eldredge were interpreted as making a very radical claim: species originate more or less overnight, in a single step, with all their new structures present. But that was a misreading. Occasionally plant species do arise in this way by hybridisation between parents of different species. But it is certainly unusual for animal species to originate in a single generation. Gould and Eldredge agree that new structures are almost always assembled over a number of generations, rather than all at once by a macromutation. Speciation - the sundering of a single lineage into two - takes generations.
In recent work, they have clarified a second misunderstanding. In claiming that species typically undergo no further evolutionary change once speciation is complete, they are not claiming that there is no change at all between generation N and generation N + 1. Lineages do change. But the change between generations does not accumulate. Instead, over time, the species wobbles around its phenotypic mean.
(Kim Sterelny - Dawkins vs Gould)
Nonetheless, if we accept the theory of gradual evolution, we can imagine many alternative forms of sexuality that could have been or could still come, even if they were highly unlikely. For example, men could ejaculate on the floor in the hope that women will sit on the ejaculate and become pregnant. Or one could imagine ejaculating on the ground as a mere display in front of women to express one's sexual fitness. We can also imagine organs shifting in function. The sexual organs would then perhaps be atrophied in their procreative function, but still, fulfill a social function of shared sensual pleasure. Procreation will continue to take place through a new organ, perhaps on the hairs that have to be entangled with those of another person. It is reported that kissing was also originally a kind of feeding, a mouth-to-mouth feeding among animals. Natural law in the context of evolution would allow us to try something new, even if the evolution is divinely guided or inspired:
Anything that is physically possible may well have been intended by the designer of the universe, who made it possible. Nature makes possible both procreation and other uses of the sexual faculties[.] (Andrew Koppelman – The Gay Rights Question)
And:
God has given us much else besides our physical and biological functions — in particular our intelligence and creativity. (Murray, T. M.. Thinking Straight About Being Gay)
Under the assumption of the theory of evolution, some things that would otherwise be immoral in traditionally bound societies could now be ethically legitimized, for example, the use of contraceptives by women and finally by men. For a precursor of sexual contraception can in principle already be found in the animal world among sperm-selecting females (the morning-after contraception pill also has parallels in animals, namely in females who eat poisonous plants after mating to kill the inseminated ovum or to hinder a possible pregnancy). All this can be found under the keyword: cryptic female choice. And there are already indications in ancient Egypt that contraceptives were used by women in the form of sponges soaked in germ-killing substances and introduced into the vagina before sexual intercourse. But for reasons of equality, men should not be disadvantaged in this respect. Another example: Originally men used their erect penis during sexual intercourse, also to scoop foreign ejaculate out of the vagina, i.e. to be able to place their sperm in a better position in the woman's body than that of other men. From an evolutionary-biological point of view, this would allow the woman to behave very promiscuously. And again out of fairness, one should not deprive men of such possible behavior. The untiringly gigantic sperm production in men and his sheer constant desire for one sexual adventure after another - i.e. his urge to always have sex with a new woman after the previous woman has lost considerable sexual attraction after a single sexual act compared to many other potential sexual partners - would speak for natural promiscuity in men. Christopher Ryan and Cacilda Jetha want to show in their book Sex at Dawn: How We Mate, Why We Stray, and What It Means for Modern Relationships how far from human nature monogamy really is.

So the assumption of evolution, whether it only takes place according to Darwinian or Lamarckian principles or both (culture could also be transferred into our nature if that is possible), makes a connection with a very specific conservative natural law ethics nearly impossible. 

A distinction between essential and accidental traits can no longer be made sharply and convincingly with regard to a genuine gradual theory of evolution. If I see myself as an instantiation of my individual form, then my hair color and the density of my hair growth are most likely essential to the form that defines me. If I see myself as a modification of an original form of life, which has a certain plasticity and instinctive impulse to live on, then everything about me is completely accidental, except for the essential qualities just mentioned. 

If I assume an exclusive, unique form for every organic individual in a population, even if all of them have a very close relationship, then I advocate a nominalistic theory of evolution. 

If I start out from a very primitive and primordial living being, I may think of it as being the essential and general nature for every subsequent living being.

A genuine species-essentialism, as the Christian theory of creation assumes and as Aristotle advocated, is therefore no longer tenable. 

Either most of my biological properties are essential or they are accidental. But this is highly problematic for natural law. For the Thomist, the essential form will either be too concrete and individual or too general and crude. There is no middle ground here. The difference between Lamarckism and Darwinism is that the former allows for a change of characteristics in the course of life, which in Darwinism is only the case in the generational transition. 

Since evolution is ongoing and somehow progressive in a natural sense (because of always changing external circumstances and conditions), so should be morality. It would, therefore, be best to already integrate future developments into today's intentions (so everything that - to put it in an astrological term - belongs to the Age of Aquarius):
Henri Bergson (1859–1941), in his book Creative Evolution, felt that you should act in a way that enables you to follow the stream of evolutionary life in the direction of the future, that you should do nothing to impede the progress of evolution. (Thompson, Mel. Ethics for Life: Making Sense of the Morals of Everyday Living)
Even if all problems of the four mentioned points were somehow magically solved, one would need on the one hand a convincing proof of God. On the other hand, the possibility of freedom of will should not only be proven but also made comprehensible conceptually. A great discussion of the problem of moral responsibility, which goes hand in hand with the problem of free will, can be found here:

https://philosophy.as.uky.edu/sites/default/files/The%20Impossibility%20of%20Moral%20Responsibility%20-%20Galen%20Strawson.pdf

Otherwise one would only have to postulate both, God and the freedom of will, and blindly rely on their existence. Since the concept of freedom of will is extremely problematic and no proof of a monotheistic God is ultimately convincing from a religiously neutral perspective, every natural law ethics is, for better or for worse, in a state of suspense. Information on the latest critiques of the Thomistic proofs of God can be found in the appendix of my other blog entry:

https://spirit-salamander.blogspot.com/2019/09/critique-of-edward-fesers-last.html

II.1) The essence of man can also be interpreted differently, perhaps even better and more convincingly, because deeper and more comprehensive than Aristotle did

1. as a blind will to live as with Schopenhauer. The main purpose here would be the maintenance of life, including reproduction. Christopher Janaway sums it up:
The will has no overall purpose, aims at no highest good. Although it is our essence, it strikes us as an alien agency within, striving for life and procreation blindly, mediated only secondarily by consciousness. Instinctive sexuality is at our core, interfering constantly with the life of the intellect. (Introduction by Christopher Janaway. In: Willing and Nothingness: Schopenhauer as Nietzsche's Educator. Edited by Christopher Janaway.)
And more from Janaway:
Whenever we undergo feelings of fear or desire, attraction or repulsion, whenever the body itself behaves according to the various unconscious functions of nourishment, reproduction, or survival, Schopenhauer discerns will manifesting itself – but in a new and extended sense. What he wants to show is that ordinary conscious willing is no different in its basic nature from the many other processes which set the body, or parts of it, in motion. Admittedly, willing to act involves conscious thinking – it involves the body’s being caused to move by motives in the intellect – but it is, for Schopenhauer, not different in principle from the beating of the heart, the activation of the saliva glands, or the arousal of the sexual organs. All can be seen as an individual organism manifesting will, in Schopenhauer’s sense. The body itself is will; more specifically, it is a manifestation of will to life (Wille zum Leben), a kind of blind striving, at a level beneath that of conscious thought and action, which is directed towards the preservation of life, and towards engendering life anew. This interesting idea is wrapped up in the much wider claim that the whole world in itself is will. Just as my body’s movements have an inner aspect not revealed in objective experience, so does the rest of the world. Schopenhauer seeks an account which makes all fundamental forces in nature homogeneous, and thinks that science is inherently unsatisfying because it always tails off without explaining the essence or hidden inner character of the phenomena whose behaviour it accounts for. His unifying account of nature is that all natural processes are a manifestation of will. (Christopher Janaway – SCHOPENHAUER: A Very Short Introduction)
2. as a blind will to power as with Nietzsche. Final Cause would be expansion and increase of one’s own sphere of power by subjugating other forces. What is good for Nietzsche? His answer:
Everything that enhances people´s feeling of power, will to power, power itself. (Nietzsche – The Anti-Christ)
Here are some passages from Nietzsche to better understand his philosophy of nature:
Wherever I found a living thing, there found I Will to Power; and even in the will of the servant found I the will to be master. He certainly did not hit the truth who shot at it the formula: ‘Will to existence’: that will—doth not exist! For what is not, cannot will; that, however, which is in existence—how could it still strive for existence! Only where there is life, is there also will: not, however, Will to Life, but—so teach I thee—Will to Power! Much is reckoned higher than life itself by the living one; but out of the very reckoning speaketh—the Will to Power!(Friedrich Nietzsche – Thus Spake Zarathustra)
And:
Assuming, finally, that we succeeded in explaining our entire life of drives as the organization and outgrowth of one basic form of will (namely, of the will to power, which is my claim); assuming we could trace all organic functions back to this will to power and find that it even solved the problem of procreation and nutrition (which is a single problem); then we will have earned the right to clearly designate all efficacious force as: will to power. The world seen from inside, the world determined and described with respect to its “intelligible character” – would be just this “will to power” and nothing else.(FRIEDRICH NIETZSCHE – Beyond Good and Evil)
Then:
Mutually refraining from injury, violence, and exploitation, placing your will on par with the other’s: in a certain, crude sense, these practices can become good manners between individuals when the right conditions are present (namely, that the individuals have genuinely similar quantities of force and measures of value, and belong together within a single body). But as soon as this principle is taken any further, and maybe even held to be the fundamental principle of society, it immediately shows itself for what it is: the will to negate life, the principle of disintegration and decay. Here we must think things through thoroughly, and ward off any sentimental weakness: life itself is essentially a process of appropriating, injuring, overpowering the alien and the weaker, oppressing, being harsh, imposing your own form, incorporating, and at least, the very least, exploiting, – but what is the point of always using words that have been stamped with slanderous intentions from time immemorial? Even a body within which (as we presupposed earlier) particular individuals treat each other as equal (which happens in every healthy aristocracy): if this body is living and not dying, it will have to treat other bodies in just those ways that the individuals it contains refrain from treating each other. It will have to be the embodiment of will to power, it will want to grow, spread, grab, win dominance, – not out of any morality or immorality, but because it is alive, and because life is precisely will to power. But there is no issue on which the base European consciousness is less willing to be instructed than this; these days, people everywhere are lost in rapturous enthusiasms, even in scientific disguise, about a future state of society where “the exploitative character” will fall away: – tomy ears, that sounds as if someone is promising to invent a life that dispenses with all organic functions. “Exploitation” does not belong to a corrupted or imperfect, primitive society: it belongs to the essence of being alive as a fundamental organic function; it is a result of genuine will to power, which is just the will of life. – Although this is an innovation at the level of theory, – at the level of reality, it is the primal fact of all history. Let us be honest with ourselves to this extent at least! – (FRIEDRICH NIETZSCHE – Beyond Good and Evil)
And:
[T]here is no more important proposition for every sort of history than that which we arrive at only with great effort but which we really should reach, – namely that the origin of the emergence of a thing and its ultimate usefulness, its practical application and incorporation into a system of ends, are toto coelo separate; that anything in existence, having somehow come about, is continually interpreted anew, requisitioned anew, transformed and redirected to a new purpose by a power superior to it; that everything that occurs in the organic world consists of overpowering, dominating, and in their turn, overpowering and dominating consist of re-interpretation, adjustment, in the process of which their former ‘meaning’ [Sinn] and ‘purpose’ must necessarily be obscured or completely obliterated. No matter how perfectly you have understood the usefulness of any physiological organ (or legal institution, social custom, political usage, art form or religious rite), you have not yet thereby grasped how it emerged: uncomfortable and unpleasant as this may sound to more elderly ears,– for people down the ages have believed that the obvious purpose of a thing, its utility, form and shape, are its reason for existence, the eye is made to see, the hand to grasp. So people think punishment has evolved for the purpose of punishing. But every purpose and use is just a sign that the will to power has achieved mastery over something less powerful, and has impressed upon it its own idea [Sinn] of a use function; and the whole history of a ‘thing’, an organ, a tradition can to this extent be a continuous chain of signs, continually revealing new interpretations and adaptations, the causes of which need not be connected even amongst themselves, but rather sometimes just follow and replace one another at random. The ‘development’ of a thing, a tradition, an organ is therefore certainly not its progressus towards a goal, still less is it a logical progressus, taking the shortest route with least expenditure of energy and cost, – instead it is a succession of more or less profound, more or less mutually independent processes of subjugation exacted on the thing, added to this the resistances encountered every time, the attempted transformations for the purpose of defence and reaction, and the results, too, of successful countermeasures. The form is fluid, the ‘meaning’ [Sinn] even more so . . . It is no different inside any individual organism: every time the whole grows appreciably, the ‘meaning’ [Sinn] of the individual organs shifts, – sometimes the partial destruction of organs, the reduction in their number (for example, by the destruction of intermediary parts) can be a sign of increasing vigour and perfection. To speak plainly: even the partial reduction in usefulness, decay and degeneration, loss of meaning [Sinn] and functional purpose, in short death, make up the conditions of true progressus: always appearing, as it does, in the form of the will and way to greater power and always emerging victorious at the cost of countless smaller forces. (Nietzsche - On the Genealogy of Morality)
Finally:
And do you know what “the world” is to me? Shall I show it to you in my mirror? This world: a monster of energy, without beginning, without end; a firm, iron magnitude of force that does not grow bigger or smaller, that does not expend itself but only transforms itself; as a whole, of unalterable size, a household without expenses or losses, but likewise without increase or income; enclosed by “nothingness” as by a boundary; not something blurry or wasted, not something endlessly extended, but set in a definite space as a definite force, and not a space that might be “empty” here or there, but rather as force throughout, as a play of forces and waves of forces, at the same time one and many, increasing here and at the same time decreasing there; a sea of forces flowing and rushing together, eternally changing, eternally flooding back, with tremendous years of recurrence, with an ebb and a flood of its forms; out of the simplest forms striving toward the most complex, out of the stillest, most rigid, coldest forms toward the hottest, most turbulent, most self-contradictory, and then again returning home to the simple out of this abundance, out of the play of contradictions back to the joy of concord, still affirming itself in this uniformity of its courses and its years, blessing itself as that which must return eternally, as a becoming that knows no satiety, no disgust, no weariness: this, my Dionysian world of the eternally self-creating, the eternally self-destroying, this mystery world of the twofold voluptuous delight, my “beyond good and evil,” without goal, unless the joy of the circle is itself a goal; without will, unless a ring feels good will toward itself-do you want a name for this world? A solution for all its riddles? A light for you, too, you best-concealed, strongest, most intrepid, most midnightly men?- This world is the will to power-and nothing besides! And you yourselves are also this will to power-and nothing besides! (FRIEDRICH NIETZSCHE – The Will to Power)
Here is a Nietzsche quote that shows what Nietzsche thinks of a restrictive natural law:
So you want to live “according to nature?” Oh, you noble Stoics, what a fraud is in this phrase! Imagine something like nature, profligate without measure, indifferent without measure, without purpose and regard, without mercy and justice, fertile and barren and uncertain at the same time, think of indifference itself as power – howcould you live according to this indifference? Living – isn’t that wanting specifically to be something other than this nature? Isn’t living assessing, preferring, being unfair, being limited, wanting to be different? And assuming your imperative to “live according to nature” basically amounts to “living according to life” – well how could you not? Why make a principle out of what you yourselves are and must be? – But in fact, something quite different is going on: while pretending with delight to read the canon of your law in nature, you want the opposite, you strange actors and self-deceivers! Your pride wants to dictate and annex your morals and ideals onto nature – yes, nature itself –, you demand that it be nature “according to Stoa” and you want to make all existence exist in your own image alone – as a huge eternal glorification and universalization of Stoicism! For all your love of truth, you have forced yourselves so long, so persistently, and with such hypnotic rigidity to have a false, namely Stoic, view of nature, that you can no longer see it any other way, – and some abysmal piece of arrogance finally gives you the madhouse hope that because you know how to tyrannize yourselves – Stoicism is self-tyranny –, nature lets itself be tyrannized as well: because isn’t the Stoic a piece of nature? . . . But this is an old, eternal story: what happened back then with the Stoics still happens today, just as soon as a philosophy begins believing in itself. It always creates the world in its own image, it cannot do otherwise; philosophy is this tyrannical drive itself, the most spiritual will to power, to the “creation of the world,” to the causa prima. (Nietzsche – Beyond Good and Evil)
3. as a blind will to die and perish as with Philipp Mainländer. The goal of life would be the weakening of all forces, and ultimately the death of both the entire organic and inorganic world. Mainländer sums up his own philosophy in this way:
1) God wanted non-existence; 2) His being was the obstacle to the immediate entry into non-existence; 3) the being had to disintegrate into a world of multiplicity, whose individual beings all have the striving for non-existence; 4) in this striving they hinder each other, they fight with each other and in this way weaken their strength; 5) the whole being of God passed into the world in a changed form, as a certain sum of power; 6) the whole world, the universe, has one goal, non-being, and achieves it by continuously weakening its sum of power; 7) each individual, by weakening its power, is brought in its course of development to the point where its striving for destruction can be fulfilled. (my own translation from German, Philipp Mainländer - Die Philosophie der Erlösung, The Philosophy of Redemption)
And:
Whoever cannot bear the burden of life, should discard it. Whoever can no longer endure it in the carnival hall of the world [...] should step out of the "always open" door into the silent night. (my own translation from German, Philipp Mainländer - Die Philosophie der Erlösung, The Philosophy of Redemption)
Finally:
The weary man who asks himself the question: To be or not to be? should draw the reasons for and against only from this world (but from the entire world: he should also take into account his darkened brothers, whom he can help, not by making shoes for them and planting cabbage for them, but by helping them to achieve a better position) - beyond the world is neither a place of peace, nor a place of torment, but only nothing. He who enters it has neither rest nor movement, he is stateless as in sleep, only with the great difference that even that which is stateless in sleep no longer exists: the will is completely destroyed. (my own translation from German, Philipp Mainländer - Die Philosophie der Erlösung, The Philosophy of Redemption)
Frederick C. Beiser talks a little more about the main idea of Mainländer (https://www.docdroid.net/Wcgd0Ga/357222834-weltschmerz-mainlander.pdf):
We live only so that we die, because the deepest longing within all of us is for peace and tranquillity, which is granted to us only in death. In this longing of all things for death, we are only participating, unbeknownst to ourselves, in the deeper and broader cosmic process of the divine death. We long to die, and we are indeed dying, because God wanted to die and he is still dying within us. (Frederick C. Beiser – Weltschmerz. Pessimism in German Philosophy 1860–1900)
And:
In the course of explaining Christian doctrine, Mainländer introduces a very modern and redolent theme: the death of God. He popularized the theme before Nietzsche, though he gives it a much more metaphysical meaning. Besides the death of God, Mainländer’s philosophy contains another signature doctrine, one no less powerful, puzzling and original. This is his idea of the death wish, i.e. that the inner striving of all beings, the final goal of all their activity, is death. At the core of everyone, Mainländer teaches us, lies their deep longing for utter nothingness. Schopenhauer’s aimless and blind will turns out to have a goal after all: death. Mainländer admits that there is an instinct for self-preservation in all of us; but he insists that, upon reflection, this desire for life is really only the means for death. We will life only for the sake of death. Mainländer finds this longing for death not only in each individual, but in the general process of history, whose sole and ultimate goal is death. (Frederick C. Beiser – Weltschmerz. Pessimism in German Philosophy 1860–1900)
And:
It is in this context that Mainländer introduces his dramatic concept of the death of God. This primal unity, this single universal substance, has all the attributes of God: it is transcendent, infinite and omnipotent. But since it no longer exists, this God is dead. Yet its death was not in vain. From it came the existence of the world. And so Mainlander declares in prophetic vein: “God is dead and his death was the life of the world”. This is Mainländer’s atheistic interpretation of the Christian trinity, to which he devotes much attention in the second volume of Die Philosophie der Erlosung. “The father gives birth to the son”—Article 20 of the Nicene Creed—means that God (the father) sacrifices himself in creating the world (the son). God exists entirely in and through Christ, so that the death of Christ on the cross is really the death of God himself. With that divine death, Mainländer proclaims, the mystery of the universe, the riddle of the Sphinx, is finally resolved, because the transcendent God, the source of all mystery, also disappears. (Frederick C. Beiser – Weltschmerz. Pessimism in German Philosophy 1860–1900)
Even physics assume something like this in the ideas of the big bang and entropy death of the universe, similar thoughts can also be found in Pandeism and in God’s Debris: A Thought Experiment, which is a 2001 novella by Dilbert creator Scott Adams; for more information look at https://www.reddit.com/r/Mainlander/ and https://www.quora.com/What-are-the-major-philosophical-contributions-made-by-Philipp-Mainl%C3%A4nder).

4. as a "seeing" will for value as with Otto Weininger. It would be a metaphysical, extra-worldly (moral and logical) value that man teleologically strives for in his innermost being. The causa finalis would thus become something transcendent.

With these four quite conceivable interpretations of the world and of man one will come to completely different moral conclusions than with those which are contained in the Thomistic natural law. Let us now take a closer look at Schopenhauer and Weininger.

The pessimist and friend of a teleological (but not theological) world view Arthur Schopenhauer interprets nature as a blind, instinctive will to live. But instead of saying that self-preservation and reproduction are moral goals, as one would expect from a natural lawyer, he comes to the opposite conclusion. The pursuit of self-preservation and reproduction is a painful process in all possible respects. This suffering can never be eliminated, not even for the shortest moment. This is simply due to the nature of the will itself as a constantly striving and never satisfiable desire, and to the fact that there are other wills in the world that inevitably thwart its goals (the failure of achieving ends and non-fulfillment of goals are, therefore, an integral part of Schopenhauer’s world). Life is therefore clearly a bad thing for Schopenhauer (the good is defined only negatively as the absence of the positive bad and evil). Unfortunately, it is impossible to avoid considering it just as bad. Accordingly, there can only be one ethical way, namely to deny the will (one’s own essence) through asceticism and quietism. Schopenhauer’s essentialism cannot, therefore, provide a direct guideline for morality. 

As a natural lawyer, you have to be a philosophical optimist in order to appear credible in the intellectual world. For even as an advocate of teleology (in a non-theological sense), as an essentialist and as a Platonist in some sense, one can, if one is pessimistic enough, strictly reject natural law with the credo "not being is better than being" or "no nature is better than nature". As a result, one would not be able to convince Buddhists of natural law. Christians who emphasize the original sin and sinfulness of nature might also turn out to be resistant if one wants to make natural law appealing to them (as stated above)

Finally, Schopenhauer only has the following to say about philosophical optimism:
Optimism, where it is not just the thoughtless talk of someone with only words in his flat head, strikes me as not only absurd, but even a truly wicked way of thinking, a bitter mockery of the unspeakable sufferings of humanity. (https://www.goodreads.com/quotes/6936544-optimism-where-it-is-not-just-the-thoughtless-talk-of)
But also the Catholic natural lawyer will not be able, strictly speaking, to stand up for the philosophical optimism needed for his theory in view of hell. No one, according to Catholic teaching, can see how far away one has already departed from God, so that no one can be sure whether he or she has made, or will soon make, a final decision against God (even if one has a blissful insuppressible feeling of confidence that one will be spared). So there is always the possibility for (every)one to go to hell. The enormous religious effort and commitment of many monks already show that from the church's point of view the probability of salvation must be very low. 

I know that Thomists say that it is better to exist in hell than not to exist at all. Of course, a non-Thomist can't believe that in the least. My non-existence before my conception was certainly not bad as such, but an eternal state of absolute agony seems really bad to me. 

The great and wise philosopher Socrates speaks positively about death as the absolute annihilation of existence: 
[I]f at death the person becomes unconscious, it will be like a very deep, dreamless sleep. And who does not enjoy that? In that case “death must be a marvelous gain”—the best rest and relaxation anyone has ever had (Apology 40c). (Ehrman, Bart D. - Heaven and Hell: A History of the Afterlife)
Here is what David Bentley Hart has to say on the subject: 
For instance, I think that traditional Thomists are entirely sincere when they argue that God could not have forborne to create souls he had predestined to eternal torment, and certainly could never now allow them peacefully to lapse again into nonexistence, on the grounds that it would constitute a kind of parsimony or jealousy on his part to withhold the gift of being—a gift he possesses in infinite plenitude—from anyone. For the Thomist, being is the first good, higher than any other, inasmuch as God himself is subsistent Being, and so, even for a soul in hell, nonexistence would be a greater evil than perpetual agony. Of course, this is ridiculous; but it helps fill in one of the gaps in the tale. A gift that is at once wholly irresistible and a source of unrelieved suffering on the part of its recipient is not a gift at all, even in the most tenuously analogous sense; and, speaking for myself, I cannot see how existence as such is truly a divine gift if it has been entirely severed from free and rational participation in the goodness of things. Being itself is the Good itself, no doubt. But, for creatures who exist only by finite participation in the gift of existence, only well-being is being-as-gift in a true and meaningful sense; mere bare existence is nothing but a brute fact, and often a rather squalid one at that, and to mistake it for an ultimate value is to venerate an idol (call it the sin of “hyparxeolatry,” the worship of subsistence in and of itself, of the sort that misers and thieves and those who would never give their lives for others commit every day). (Hart, David Bentley - That All Shall Be Saved)
The only will that is not blind compared to the ones mentioned above is the conscious will to value conceived by Otto Weininger. For Weininger in his book SEX & CHARACTER, the "birth of Kant’s ethics" is the "most heroic act in world history". Kant’s Critique of Practical Reason is "the most sublime book" and there is "no other ethics" than that of Kant. Most Thomists do not understand that Kant’s ethics follows the tradition of Plato (and Plato is highly valued by the Thomists) and Saint Augustine. It is above all Kant’s concern to determine the metaphysical idea of the good, and he tries to do this more precisely than his predecessors. Thomists like Edward Feser seem to be very superficial when it comes to Kant’s assessment, which Weininger accuses many of:
In Kant’s ethics, nothing is understood as little as the demand to act according to a most general maxim. One still thinks one has to see something social in this, Büchner’s ethics (“What you don’t want people to do to you”, etc.), a guideline for a penal code. (Otto Weininger - SEX & CHARACTER
(http://www.fatuma.net/text/sexchar.pdf)
As Schopenhauer summarizes Kant’s ethics quoting Kant, it shows why the Kantians cannot become natural lawyers:
That moral law [...] is in addition supposed to be a law cognizable a priori, independent of all inner and outer experience, ‘resting solely on concepts of pure reason, it is supposed to be a synthetic principle a priori’: directly connected with this is that it must be merely formal, like everything that is cognized a priori, and so must relate merely to the form, not the content, of actions. [...] [Kant] expressly adds that it ‘must not be sought in the nature of the human being (the subjective) or in the circumstances of the world (the objective)’ and, that ‘it must not borrow the least thing from acquaintance with the human being, i.e. from anthropology’. Again he repeats ‘that one should not let oneself think of deriving one’s moral principle from the special constitution of human nature’; similarly: that ‘Everything that is derived from a special natural constitution of humanity, from certain feelings and propensities, and even, if possible, from a special tendency that would be peculiar to human nature and would not necessarily have to hold for the will of every rational being’ could yield no basis for the moral law. (Arthur Schopenhauer – The Two Fundamental Problems of Ethics)
Weininger interprets Kant as follows:
Truth, purity, faithfulness, uprightness, with reference to oneself; these give the only conceivable ethics. Duty is only duty to oneself, duty of the empirical ego to the intelligible ego. These appear in the form of two imperatives that will always put to shame every kind of psychologism – the logical law and the moral law. The internal direction, the categorical imperatives of logic and morality which dominate all the codes of social utilitarianism are factors that no empiricism can explain. (Otto Weininger – SEX & CHARACTER).
And:
Logic and ethics are fundamentally the same, they are no more than duty to oneself. They celebrate their union by the highest service of truth, which is overshadowed in the one case by error, in the other by untruth. All ethics are possible only by the laws of logic, and logic is no more than the ethical side of law. Not only virtue, but also insight, not only sanctity but also wisdom, are the duties and tasks of mankind. Through the union of these alone comes perfection. (Otto Weininger - SEX & CHARACTER)
Kantian morality is in any case inseparably linked with logic and not dependent on any contingent natural phenomena such as those of one’s own body. Weininger also tries to justify the "the demand for the sexual abstinence on the part of both sexes":
Coitus is immoral because there is no man who does not use woman at such times as a means to an end; for whom pleasure does not, in his own as well as her being, during that time represent the value of mankind. During coitus a man forgets all about everything, he forgets the woman; she has no longer a psychic but only a physical existence for him. He either desires a child by her or the satisfaction of his own passion; in neither case does he use her as an end in herself, but for an outside cause. This and this alone makes coitus immoral. (Otto Weininger – SEX & CHARACTER)
Weininger’s ethics is summarized as follows:
But it is only he who feels that every other man is also an ego, a monad, an individual centre of the universe, with specific manner of feeling and thinking and a distinct past, he alone is in a position to avoid making use of his neighbours as means to an end, he, according to the ethics of Kant, will trace, anticipate, and therefore respect the personality in his companion (as part of the intelligible universe), and will not merely be scandalised by him. (Otto Weininger – SEX & CHARACTER)
If man’s essence were purely spiritual, then it is clear that animal impulses should be given as little space as possible to unfold, that is, in this respect, only the most necessary for life should be done so as not to endanger one's morality. In sexuality, even if it should implement all the guidelines of Thomism, the metaphysical idea of what is good, according to Weininger, is violated to a high degree. As a pessimist and spiritual antinatalist, Weininger could not have done anything with natural law:
Now for the first time, looking at the woman question as the most important problem of mankind, the demand for the sexual abstinence on the part of both sexes is put forward with good reason. To seek to ground this claim on the prejudicial effects on the health following sexual intercourse would be absurd, for any one with knowledge of the physical frame could upset such a theory at all points; to found it on the immorality of passion would also be wrong, because that would introduce a heteronomous motive into ethics. St. Augustine, however, must certainly have been aware, when he advocated chastity for all mankind, that the objection raised to it would be that in such a case the whole human race would quickly disappear from the face of the earth.
This extraordinary apprehension, the worst part of which appears to be the thought that the race would be exterminated, shows not only the greatest unbelief in individual immortality and eternal life for moral well-doers; it is not only most irreligious, but it proves at the same time the cowardice of man and his incapacity to live an individual life.
To any one who thinks thus, the earth can only mean the turmoil and press of those on it; death must seem less terrible to such a man than isolation. If the immortal, moral part of his personality were really vigorous, he would have courage to look this result in the face; he would not fear the death of the body, nor attempt to substitute the miserable certainty of the continuation of the race for his lack of faith in the eternal life of the soul. The rejection of sexuality is merely the death of the physical life, to put in its place the full development of the spiritual life.
Hence it follows that it cannot be a moral duty to provide for the continuance of the race. This common argument appears to me to be so extraordinarily false that I am almost ashamed to meet it. Yet at the risk of making myself ridiculous I must ask if any one ever consummated coitus to avoid the great danger of letting the human race die out, if he failed in his duty? And would it not follow that any man who prefers chastity would be open to the charge of immoral conduct? Every form of fecundity is loathsome, and no one who is honest with himself feels bound to provide for the continuity of the human race. And what we do not realise to be a duty, is not a duty.
On the contrary, it is immoral to procreate a human being for any secondary reason, to bring a being into the limitations of humanity, the conditions made for him by his parentage; the fundamental reason why the possible freedom and spontaneity of a human being is limited is that he was begotten in such an immoral fashion. That the human race should persist is of no interest whatever to reason; he who would perpetuate humanity would perpetuate the problem and the guilt, the only problem and the only guilt. The only true goal is divinity and the union of humanity with the Godhead; that is the real choice between good and evil, between existence and negation. The moral sanction that has been invented for coitus, in supposing that there is an ideal attitude to the act in which only the propogation of the race is thought of, is no sufficient defence. There is no such imperative in the mind of man; it is merely an ingenious defence of a desire, and there is the fundamental immorality in it, that the being to be created has no power of choice with regard to his parents. As for the sexual union in which the production of children is prevented, there is no possible justification.
(Otto Weininger - SEX & CHARACTER  http://www.fatuma.net/text/sexchar.pdf)
As one can see, there are many alternatives to determine man’s primary nature (which controls all secondary individual biological functions and faculties) and derive an ethics from it (provided one has no problem with naturalistic or moralistic fallacies), and probably some more than those mentioned here. I have only referred to radical alternatives, which I do not support myself. But by thinking of them as possible, they put any other ethical concept or theory into perspective. And none of these ethics, as we have seen, would ultimately be similar to the rest.

II.2) The basic idea of natural law regarding sexuality is not very plausible

The Catholic Thomists confidently assert the following sentence, which is of great ethical importance to them: Every sexual act (a sensual action involving a possible ejaculation) must have only procreation as the indispensable primary purpose! (we will only talk about potentially fertile sex here, not sterile sex, so as not to complicate things unnecessarily). For them, this sentence is apparently self-evident and immediately obvious as an axiom, in contrast to the Catholic theologian Grisez, for whom that sentence itself is neither obvious nor easy to prove. In any case, according to the Thomists, the main purpose of sex is reproduction, and any other use of one’s sexual abilities, for example for foreplay, fun, or emotional attachment, is secondary. While the Thomist may call what falls under this secondary use purposes, they are only permitted optional means to the sole purpose of reproduction, in other words, they are only accidental accessories.

But how can I be sure that ejaculation can only take place in the vagina for the purpose of reproduction and nowhere else? How can I completely rule out the possibility that there are other nature-given primary purposes of sexuality? The fact that the ejaculate in the vagina often, but not too often (actually rather rarely), causes reproduction does not prove that there is only one ultimate natural purpose of the sexual act.

There may be other natural forms of sexuality outside the context of reproduction that serve other primary natural purposes. The Thomist basically cannot logically rule this out. At most he could say: It may well be that there is one or another purpose of this kind. Since I cannot see them myself under any circumstances, but only clearly see the one that ultimately serves reproduction, I do not want to take any unnecessary moral risk. All well and good. However, if such forms of sexuality do exist, one must admit that the realization of their specific purposes does not automatically lead to the frustration of the reproductive purpose. For this one purpose is no longer important at all at the moment when another is aspired to and realized. The purpose of reproduction in such a case would be, if at all, only secondary and optional.

You can understand all this better if you look into the animal kingdom, which Aquinas also likes to do. In many cases, the sexual behavior of animals is not only about reproduction. The least that can be said on this subject is that sexual stimulation that any animal carries out on itself or that is caused by members of the same species (whether heterosexual or homosexual) is quite common and normal, without it leading to reproduction. And sometimes the "sodomitic" activities of the animals even lead to a wasted ejaculation, as with orangutans.

Especially interesting is the case of a squirrel species living in desert areas, the Cape Ground Squirrels, whose males occasionally perform autofellatio. Here is the respective link:


After ordinary copulation with a female, the male satisfies himself orally and swallows his own semen. The purpose of this procedure, according to the researchers, is to clean the seminal and urethral tubes. An infection is thereby to be avoided.

One can now make a simple thought experiment with that squirrel, which is just satisfying itself sexually, by giving it from outside a spirit, which, according to Aristotle, also comes "from outside (sic)" into human beings. Thus the male with his new consciousness can encounter moral mines in every one of his actions. After ejaculation, however, one cannot accuse him of having violated the purpose of reproduction and thus sinned. This would be ridiculous. Reproduction (and ejaculation in the vagina as the main means to do so) really didn’t play a role at that moment. The purpose of cleansing was given alone and decisive.

Let the female bonobos also have a rational mind and ask them if they would thwart something in their nature by rubbing their clitoris together to orgasm. From their point of view, the sexual behavior in which they engage would reduce social tensions and build up social bonds. That would be more than justified teleologically. For Aquinas, too, there is a higher purpose of sociability.

If one wants to object here that the animal world is not suitable as a possible model for a foundation of an ethics (under the aspect of evolution it is indeed possible), then Aquinas must also not look into the animal world, then he must look more closely into the anatomical and physiological nature of man with its functionalities, into that nature, then, which, as Andrew Sullivan puts it, doesn’t really care whether the male seed is, under any circumstances, wasted (that’s the unavoidable conclusion of summarizing all the empirical facts). This is what Andrew Sullivan has to say about this:
Let’s posit a perfect, Catholic married couple who live their life according to natural law in every respect. They never use contraception and never engage in any sexual act that does not result in the penis depositing semen in a vagina. Assume further that both are fertile. If the woman conceives, then it must follow that the point of depositing semen in the vagina is literally irrelevant for the following nine months or so. Exclusively procreative sex, in other words, naturally necessitates continual nonprocreative sex. If the husband were to refrain from any sexual activity in those nine months, to avoid activity “contrary” to nature, he wouldn’t stop producing sperm. Even if he did not masturbate, his body would emit sperm in nocturnal emissions, triggered by erotic desire programmed into his brain. In other words, his body itself would spontaneously and without any intention on his part act in a way that is “contrary” to the natural law. It’s hard to see how such a phenomenon can be deemed “contrary” to any reasonable account of “nature.” In fact, any reasonable account of human sexuality, observed by any rational person, would conclude that, even under the strictest of Catholic moral teachings, most sexual activity between a man and wife must necessarily be nonprocreative. A man’s sex drive does not disappear the moment his wife becomes pregnant, and neither does his wife’s. Put it another way: If the purpose of the penis was solely procreation with one monogamous female partner, as the natural law philosophers insist, it would surely have been designed very differently. It would ejaculate much more rarely and be attuned to another human’s menstrual and reproductive cycle. To take this thought experiment still further, it might even be designed only to ejaculate by some mechanism within the vagina and by no other means. In fact, of course, the male sexual organs produce an almost infinite number of sperm, an infinitesimal amount of which will ever become another human life. It seems odd that when a phenomenon occurs to one sperm out of millions, the one should be seen as the rule and the millions the exception. Moreover, the penis can be stimulated by almost any other physical object, and at its sexual peak, may ejaculate countless sperm several times a day. Why would this be a fact of our physical nature if the sole purpose of a penis is to procreate with one other human being? These are not arguments. They are facts. If nature implies a purpose, then the purpose of the male sexual organs posited by natural law is obviously divorced from its actual, empirically observed function. (Andrew Sullivan – The Conservative Soul)
The Christian Robin Gill also describes that certain events preventing reproduction can be natural since it occurs in many natural ways:
Further, non-procreative, nocturnal emissions of semen, infertility at certain points in the menstrual cycle, and abortions in the form of non-implantations or miscarriages, all happen spontaneously and frequently and thus might be regarded as ‘natural’. (Robin Gill - A Textbook of Christian Ethics)
If you take the perspective of a single sperm, it all becomes clear. Sperm and eggs are the main protagonists of sex. Their main goal is to merge. Everything else like the penis, vagina, penetration, and lust are only means to an end. If we do not seek out sexual partners, we do not give a single sperm a real chance of achieving its goal. Because even in the testicles, all sperm still has the primary goal of penetrating into an egg cell. However, they have a kind of barrier in front of them and fidget back and forth with impatience, especially as they won’t live too long. It simply cannot be avoided that the goal of millions of sperm and many eggs is frustrated. If the much-vaunted teleology of sexuality is ultimately found only in the individual sperm, then one must be puzzled because of the nature, physiology, and anatomy of the human body (being), which unconsciously aims to thwart the goal of most sperm. Strangely enough, I'm not allowed to consciously thwart that goal.

Here is a quote that echoes what has been said: 
There are plenty of biological systems that only rarely fulfill the purpose for which they were selected. Think of the sperm cell: its raison d’être is to fertilize an ovum. And yet, the percentage of sperm cells that actually manage to pull this off is vanishingly small. […] Sperm cells in normal environments rarely realize their biological destiny function[.] (David Livingstone Smith - Why we lie: the evolutionary roots of deception and the unconscious mind)
Mimicking failure is said to be a moral problem, although failure is naturally the rule. 

John T. Noonan can also contribute something interesting, and draw an equally interesting conclusion:
At the most on four days a month is the union of intercourse and fertility normal. If we seek to understand the divine plan from what nature has given humanity, we must infer that it is for a brief part of any life that fertility is intended, and that nature has designed man so that many acts of intercourse will be sterile. 
[...]
The reason a sterilizing act is wrong is that it asserts man's dominion over the generative process and effects the disruption of the natural nexus. But when steps [with contraceptives] are taken to assure that intercourse is not fertile in a period not intended by nature to be fertile, man acts in subordination to the divine plan and does not effect any disruption of the sacred link between love and fertility. Further, the directly intended act is the assurance of natural sterility. 
[...]
But at those times when nature intends no procreation, there is no interference in the structure or signification of the sexual act when means [contraceptives] are used increasing the probability that the natural rhythm will hold. (John T. Noonan, Jr. - Contraception A History of Its Treatment by the Catholic Theologians and Canonists)
Or Aquinas doesn’t even look empirically into human nature at all, and, like Kant, tries to derive an ethics a priori only from the intellectual soul, the conception of which, however, is somewhat confused in the case of Aquinas.

Is it nevertheless possible to transfer by analogy to humans what applies in general to some animals, such as the squirrels or bonobos mentioned above? I do not mean to say that these special cases should be applied to humans; it is only a question of the basic idea, which is legitimate even without examples from the animal realm. More specifically, can a natural faculty have a different natural and ultimate purpose in another context or under other circumstances, so that in the end there is a different natural primary purpose depending on the context? Can different situations lead to different primary purposes of a natural thing? Can there be a case where something is an obligatory purpose in one context but only an optional one in another? Can the Thomist rule this out with absolute certainty and declare it impossible?

If one admits the theory of evolution, the claim of impossibility will hardly be justifiable anymore. Nor can anything be ruled out by defending Aristotle’s principle that everything that applies to artifacts (a single knife to eat, hunt, to be admired just hanging on the wall above the bed, trade, carve, threaten, shave, carve, kill, show off, and to be melted, depending on the situation) can be legitimately applied to all natural things.

By the way, do Thomists actually give examples of natural faculties that have more than one main purpose (a different context for each main purpose)? If so, that would be very interesting. Because then you could ask how they came up with it and why they can so rigorously rule it out in sexuality. I’m talking about the main purposes here so that no misunderstandings arise.

Even Kant, who is rigorous in moral matters and thinks deontologically grants due to the power of context a casuistry a systematic place in the building of his ethics and devotes himself to casuistic questions, to what extent exceptions to the strict rule, e.g. in the question of suicide, white lies, the enjoyment of wine, can be permitted under certain circumstances. Aquinas also can lose his moral rigor from time to time:
It is true that Aquinas did also appear to hold some absolute moral rules, such as the one that disallowed lying … but this is not what is stressed in the account of natural law … His overall position is that there are what are called ‘primary precepts’ which are exceedingly general (such as the duty to worship God, and to love one’s neighbour) and ‘secondary precepts’ which are more specific, such as the duty to have only one husband or wife. However, the secondary precepts all have to be interpreted in the context of the situation, and it is here that the flexibility of natural law arises. At one point [Aquinas] argues as follows: ‘The first principles of natural law are altogether unalterable. But its secondary precepts … though they are unalterable in the majority of cases … can nevertheless be changed on some particular and rare occasions …’ … Aquinas argues, ‘The more you descend into the details the more it appears how the general rule admits of exceptions, so that you have to hedge it with cautions and qualifications.’ This is an important qualification and shows that there may be more flexibility in the natural law approach than is often supposed. It may also open the door to a natural law approach to morality coming together with situation ethics (see ch. 10) – for instance through a form of proportionalism. Whereas Aquinas is firm in his insistence on the primary precepts of natural law, he seems to show more flexibility when discussing the secondary precepts which ‘unpack’ these and sometimes modern supporters of a natural law approach to ethics do not sufficiently recognise this. (Vardy, Peter - The Puzzle of Ethics)
In my opinion, besides reproduction, one could assume three other equal (in the sense of a hierarchy) real purposes of sexuality (even if the name of sexuality may no longer be appropriate in the new context), which do not seem abstruse or farfetched, but correspond to a certain common sense. One could also name others like spiritual exaltation currently performed in Hindu cultures and also in Western cultures or sex as a test of whether one fits bodily and emotionally with the other person at all. This was said by Bertrand Russel and it seems very reasonable and rational to me.

Here's what Russell says:
I should not hold it desirable that either a man or a woman should enter upon the serious business of a marriage intended to lead to children without having had previous sexual experience. There is a great mass of evidence to show that the first experience of sex should be with a person who has previous knowledge. The sexual act in human beings is not instinctive, and apparently never has been since it ceased to be performed a tergo. And apart from this argument, it seems absurd to ask people to enter upon a relation intended to be lifelong, without any previous knowledge as to their sexual compatibility. It is just as absurd as it would be if a man intending to buy a house were not allowed to view it until he had completed the purchase. (Bertrand Russell - Marriage and Morals)
Slavoj Žižek makes an interesting comment on the spiritual aspect: 
This is why the Catholic argument that sex without procreation, whose aim is not procreation, is animal, is wrong: the exact opposite is true, sex spiritualizes itself only when it abstracts from its natural end and becomes an end-in-itself. Slavoj Žižek - DISPARITIES
And:
This is why the Catholic Church tends to downgrade sexual activity (when it’s separated from procreation) to raw animality: it intuits very accurately that sex is its great competitor, the first and most basic experience of a properly meta-physical experience. Sexual passion introduces a violent cut into the flow of our daily life: another dimension intervenes into it and makes us neglect our daily interests and obligations. The Catholic Church ignores the blatant fact that it is precisely animals which do it for procreation, while humans in the thrall of sexual passion isolate something (sexual act) that in nature functions as a means of procreation and make it a goal-in-itself, an act of the intense experience of spiritualized enjoyment. (Slavoj Žižek - Sex and the Failed Absolute)
Sex as a natural ability could be compared to a Swiss army knife. This knife has many primary functions. For example, there is a specific knife, say a bread knife. There is also a magnifying glass for visual enlargement and a bottle opener. When one function is activated, it doesn’t have to be the same for the other functions. I don’t need a bottle opener to cut bread.

I completely agree with what Robin Gill says:
Even if it is conceded that procreation is the obvious function of sexuality, it is far from clear that it should be the only, or the indispensable, function of human sexuality. (Robin Gill - A Textbook of Christian Ethics)
The three (possible) natural (ultimate) purposes each take place in their own context, the contexts of pubertal adolescence, love display, and health promotion. And what has been said before would also apply to them, namely that as soon as one of these primary purposes is pursued and fulfilled, the others do not play a role because they do not exist as something (morally) important at this moment and in this context, but can at most be considered coincidentally and by the way.

In addition, for logical reasons, an activity can only have one particular final cause at a time and in a particular context. The proposition of contradiction excludes that several primary purposes can be assigned to one and the same thing at the same time. That could perhaps be explained in more detail like that: If sexual activity is understood as a coherent process, then, in fact, there cannot be more than one primary purpose at the same time. For a better understanding, we can compare the human being with a cube and the surface of the cube with the number 6 on it would stand for sexual activity. Different primary purposes should be represented by different colors on that side of the cube. Reproduction would be green, then all means for that very end should also be green. For they serve only the whole green purpose. When other colors are applied, the whole side is no longer represented by a single purpose. The intention would be dispersed and scattered. That means there could be parts that work against this purpose and undermine it. The whole surface must be green and not be fragmented by applying other colors. If at all, another color should be applied completely, such as red for the display of love, and propagation, if it is desired as a means, should also be colored red. Alternatively, the contraceptive is to be colored red.

Now I come to the three possible main purposes, which always contain an innocent intention and do no harm, not even in the slightest sense (on the contrary, one benefits and gains a lot):

1) Personal maturity: 

If you want to start a family responsibly, you should definitely have reached a certain degree of maturity beforehand. Everyone would agree on that. It is indisputable today that the process of personal maturation, along with many other very important factors, encompasses all sexual activities, including sexual activities that are not related to marriage or propagation. It is therefore expressly stated that "sexual development is part of personality development and begins with birth". Experts also say (you should listen to experts as authorities, according to a Thomist like Feser) that psychosexual development cannot be separated from cognitive maturation processes. Even in infancy, a lot happens in the sexual sphere, which Freud recognized at his time. Dry orgasms come about through masturbation or randomly fitting body positions, both in boys and girls. With the onset of puberty, voluntary and involuntary ejaculations occur increasingly in young boys at night, during which nature itself, by the way, thwarts its own purposes on a gigantic scale (Feser gladly personifies nature in his speech; in addition, nature is ultimately the activity of God). Then one matures above all in contact with other children, which also includes the encounter with the opposite sex, an encounter which will inevitably have a sexual touch every now and then. Feser could hardly deny that one has to bring a certain personal maturity into a marriage in order for it to become reasonably stable. However, many man-boys may have to "break their horns" before they are ready for the bond of marriage. I don’t think Feser will want to say that a thirteen-year-old boy who heavily "makes out" with a thirteen-year-old girl is immediately committed to reproduction and marriage, just because the boy may have ejaculated in front of the girl's vagina covered by thin panties.

It’s clear that this can’t have a real future and is only part of a Coming of Age process (and not of a procreation process!). Nature has "theoretically" already given them the physical sexual maturity and a minimum of intelligence for a marriage, but the personal maturity is still completely lacking. When this personal maturity is attained one day, it disappears from the stage as a final cause, so to speak. Until then, however, much could be safely subordinated to it. Here is a sketch of man’s sexual development, in the phases of which one can hardly speak of violations against natural law:
Up to 2 years: Genital manipulation; erection in boys; experience of pleasant genital feelings; enjoying nudity
3 to 5 years: Pleasant masturbation, sometimes up to orgasm; sexual games with peers and siblings: showing one’s own genitals; exploration of one’s own genitals or those of other children; enjoying nudity; undressing in the presence of others.
6 to 12 years: sexual games with peers and siblings; role plays and sexual fantasies; kissing, masturbation, simulated sexual intercourse; shame and embarrassment; sexual games are kept secret from adults; being in love and lovesickness; interest in sexuality in the media; beginning of pubertal changes: Menarche and breast development in girls; ejaculation in boys
13 years and older: continuation of physical change; kissing, petting
[petting is in principle a separate category that does not include reproduction], mutual masturbation; sexual fantasies and dreams; sexual intercourse (found on a German psychology site)
2) Love: 

Man is perhaps mainly a spiritual, emotional, and erotic being and secondarily an animal and lustful one. The spiritual, emotional, and erotic union between two people that can happen in the sexual act, but does not have to, is an expression of true love and affection. This noble purpose of a union is also recognized by Christians who then refer to a passage from the Bible in Matthew 19:5-6: "So they are no longer two, but one flesh."

The unifying love purpose can be considered either as fully equal to the reproductive purpose or only secondary like any other tender action that does not directly serve the reproductive process. However, if it were merely secondary, it would either have to be completely negligible, so that it would not matter whether married couples love each other at all. Or it is an obligatory means to the end of reproduction like ejaculation in the vagina. I don't think either would be in the Thomist's sense. Ergo, the purpose of love is on an equal footing. It should be mentioned that the Catholic Church claims the article of faith that both purposes (love and propagation or fertility) are one and the same. As a non-Catholic, I don't see it that way. 

From a Catholic couple, who is bound to each other forever by a single sexual act, but who since this action cannot stand each other at all, but nevertheless does not want to renounce sex, one cannot demand the spiritual, emotional and erotic purpose of union. It is well known that love cannot be forced. Anything else would be simply and poignantly absurd, whereby that marriage is already based on an absurd basis. If anyone objected: Why do you have to use contraception if you want to show your love, then I could say: pregnancy and a baby would (at least for the time being) be a love inhibitor or reducer or stopper or even destroyer because in many cases deep romantic love ends with a child who now gets all the attention. As soon as children enter, men and women are forced to realize that their feelings towards each other are no longer what is of most importance. So those feelings fade. 

Perhaps the Thomists secretly and in the background assume the false psychological idea that non-procreative contraceptive sex is purely selfish as opposed to procreative sex. But the great desire of man and woman for children is anything but selfless.

And those who defend contraception "claim that contraceptive intercourse can have the same good purposes as other licit though unfruitful sexual relations." (Germain Grisez - Contraception and the New Natural Law)

And what's more, man can have intercourse simply out of love, unlike animals. This is what distinguishes him/her as human. But showing love does not necessarily always have to be about sexual intercourse. Oral sex could be just as suitable (there's definitely a disposition to satisfy one's sexual partner orally).

Here is one more interesting addition to the unitive function (what was a comment on an interview on condom use in Catholic couples when one partner has HIV and the other does not):
However, I want to go back a step, and challenge the definition that the act of "union" (for unitative ends) is when semen hits cervix.
Why is the male but not the female gamete relevant? Isn't this definition based on the ancient but incorrect idea of conception as the planting of male "seed" in the fertile ground of the cervix? Surely even in cases of aspermia, the act is unitative?
I would want to redefine the unitative moment as a mental/spiritual moment e.g. the moment of orgasm. The use of condoms therefore ceases to be "evil" or even relevant. They do not prevent the unitative end from being attained (which is one "Good"), only the generative!
(Posted by: Jim Vaughan | August 15, 2011 at 12:00 PM)

https://philosophybites.com/2011/07/luc-bovens-on-catholicism-and-hiv.html
3) Health: 

Sex has been proven to be healthy for body and mind. It strengthens the immune system, it counteracts depression and reduces frustration and even prolongs life:
It has been shown several times in studies that people in happy relationships with a fulfilled sex life get older than others[.] (from a German newspaper) 
This alone represents a higher purpose than mere pleasure and fun. Aristotle also acknowledges the sole purpose of health under certain circumstances (consequently in a certain context) when he says:
[M]en who are too old, like men who are too young, produce children who are defective in body and mind; [...] [Couples where the man is over 54] should cease from having families; and from that time forward only cohabit with one another for the sake of health; or for some similar reason. (Aristotle -Politics Book Seven Part XVI)
So "only" for the sake of your health. Aristotle says so straight out. The word "cohabit" clearly refers here to sexual intercourse. As Aristotle says, making children at a certain age when one is too young or too old, had a negative influence on the development of the children. Therefore, if they had existed at that time, he would have spoken out in favor of the use of contraceptives in certain cases (bad health, overpopulation).

And also in Thomistic natural law, self-preservation, by which one must primarily understand the preservation of one’s own health, is a higher purpose to which one can temporarily subordinate or sacrifice various bodily functions (use of catheters, artificial nutrition, plaster application, etc.). If I were healthy, I wouldn't be allowed to do all this. Feser, for example, brings only a radical example of a necessary amputation of a limb to stay alive. This means, for example, that I cannot have a kidney removed from me to feed my dogs. That would be a violation of natural law. But if a kidney has to be removed to prevent the spread of a serious disease, then there is no violation, not even in the act of removal itself. But what would the Thomist say about a semen sample that is absolutely necessary for a vital examination? He can only say "permitted" or "not permitted". When he says the former, he definitely makes a big concession.

According to a friend of mine, using a treadmill in the gym would frustrate the primary purpose of walking, which is to get from A to B. On the treadmill, however, you stay on the same spot. Just as the ejaculation goes into the void during coitus interruptus, every step of my legs on the treadmill goes into the void. The person training would say he or she is doing it for health reasons. The workout context makes the supposed main purpose of a faculty vanish from one’s sight, as it is not even aspired to. One works only against atrophy of the ability.

There can be no objection to the preservation and care of physical/mental faculties through pure activity and exercise solely under the aspect (purpose) of general (physical and mental) health, the aspect of a health consciousness that considers the soul as a form completely in the whole body and completely in each of its parts.  

Why should what has been said not apply to all human abilities, for health is ultimately the most fundamental requirement for the realization of any ability or organ? Urologists even recommend regular and moderate masturbation to keep those parts of the body intact that are relevant for reproduction or, more generally, for the proper functioning of the genitals. Such statements from experts are found in great numbers, so I do not need to quote anything.

Other examples: Although chewing low-nutrient chewing gum abuses the chewing movement and cheats the stomach, it is okay because it promotes a healthy mouth flora, keeps the chewing muscles in good condition, and increases the ability to concentrate. The ability to use one’s own body as a weapon against other people is then purposefully activated when I have to take up self-defense in a real situation, make a criminal unable to fight, or kill an opponent in war. The goal, which is fixed with every form of self-defense, is not even aimed at in combat training and is therefore never realized. Still, I’m not doing anything wrong. The context is simply a different one.

With sexuality, as it looks to natural lawyers, it seems that it is not just about a possible perversion of a natural ability, but above all about the importance of a function being at stake. How do I know which function is very important? Wouldn't it then be reprehensible to prevent pain sensations through medication, since pain generally has a very important function in a person's life?

The following can be interesting. A British surgeon in the 19th century named James Esdaile was one of the first to use hypnosis as an anesthetic during operations. But religious colleagues had a problem with it and sued him because it did not correspond to their, let me say, divine natural law intuition.
During the trial one physician claimed that Esdaile was sacrilegious, because God meant for man to feel pain and Esdaile was preventing this with hypnosis. (Leslie M. Lecron - Complete Guide to Hypnosis)
Even if those three other main purposes I have described in detail do not naturally exist, they could imply morally neutral "other than" uses instead of immoral "contrary to" ones. According to natural lawyers, I can act in two ways or two modes, neither of which is in accordance with the natural end of the faculty relevant to those two possible actions. In both actions, which can be extremely short in time, the natural purpose of the faculty I have to use for these two actions to get going is not sought and therefore never realized (instead, an artificial purpose is sought and perhaps realized). But in one case there is moral neutrality (other than), in the other one we are dealing with moral evil (contrary to). If this distinction is not tenable, the whole theory of natural law in the sense of most Thomists like Edward Feser would collapse. They would admit that, too. Both cases are at first "other than" cases, but in the second case, something bad comes along. This will be discussed again later. But you should already look at the opening question of this page to better understand the problem:

https://philosophy.stackexchange.com/questions/46909/the-perverted-faculty-argument

Through the aforementioned distinction, Feser can defend himself against obvious criticism like this (provided the distinction really makes sense):
Critics object that it needn’t be wrong to use organs for something other than their primary biological purpose; for example, there’s nothing wrong in using our feet to kick a football. So it needn’t be wrong to use sex organs for something other than their reproductive functions. (Harry J. Gensler - Ethics: A Contemporary Introduction, Chapter 13)
Here is an addition to the soccer problem, which could make sense if Feser's distinction between "other than" and "contrary to" is questionable:
Consider the human foot. If the human foot has any proper purpose it would be for walking and standing. The human foot is a very important feature of human anatomy and has an important role in human evolution. One very important, though not so obvious, anatomical difference between apes and humans is that we have flat feet that are well suited for walking long distances. Chimps and other apes, by contrast, have feet better suited for hanging from trees than for walking. That is why early humans (Homo erectus) wandered to every corner of Europe and Asia, while our chimpanzee cousins never strayed from tropical Africa. Does that mean that soccer players or karate enthusiasts, who also use their feet for kicking, are misusing their foot organs in ways that are morally objectionable? That is absurd. Since kicking soccer balls, or planks of wood, is obviously not morally wrong, the teleological principle fails. One reply that defenders of the teleological argument might give is that kicking is included in the many different functions or purposes that the foot has. They might then claim that the proper purpose of the foot is standing and walking, and kicking. But this list would have to be extended indefinitely to include all the uses of the foot that are obviously not morally suspect. The proper purposes of the foot would include walking, standing, kicking, hopping, pedaling a bicycle, tap dancing, manipulating a wah pedal, crushing cockroaches, and so on. But some of these allegedly proper functions are quite implausible. The foot could not have evolved for bike riding since the bicycle has only been around for about 150 years. Moreover, if the teleological view is to be meaningful, there must be some use of the foot that is not included in its repertoire of innate, natural functions. Suppose I learn to write or draw with my foot. The manipulation of writing tools is surely not included in the innate, proper function or purpose of the foot. (The human foot is not well suited for tool use and its selection had nothing to do with being useful for that purpose.) The teleological principle would imply that I am doing something morally wrong by writing with my foot. That is obviously unreasonable. If the foot has more than one purpose, then there is good reason to think that the genitals also have more than one purpose. And if it is not morally wrong to use a foot for something other than its purpose, then the same should be true of the genitals. Defenders of the teleological argument against homosexuality could try to argue that the purpose of the foot has no moral significance, but the purpose of the genitals does. But that would be ad hoc, meaning that this distinction would be proposed only because it is convenient for those who are trying to argue that homosexuality is morally wrong and not for any independent reason. (Chris Meyers – THE MORAL DEFENSE OF HOMOSEXUALITY)
Now one can juggle a little with natural law principles regarding the walking ability, the purpose of which is to get from A to B and also to promote self-preservation (because the walking ability may only be a part of the self-preservation system). Here are two examples of misappropriation:

1. Treadmill: Parts of the walking faculty are properly activated, but one remains (pervertedly) on the spot.

2. Soccer: Parts of the running ability are properly activated and the positions are constantly changed. But legs and feet aren't for kicking a soccer ball (they are only very shortly perverted when kicking but still perverted).

Treadmill use and soccer do not, however, directly serve self-preservation, the higher goal of the walking faculty, so they should be considered as perverting. Both movements, for example, are not directly used to procure food. Or do they instead serve the courtship display directly and thus procreation?

Is self-preservation in the hierarchy of ends above procreation? This is a contentious point. If self-preservation takes precedence over reproduction, then oral sex, if it is stimulating or relaxing for the whole organism (kind of self-preserving), could be morally completely legitimate. What is there for whom? Personally, I would say: Reproduction (the reproductive faculty) is only a function of self-preservation. In relation to higher ends, one can understand the parts of a faculty in different ways, as well as the faculties themselves. And this fact might lead to all sorts of argumentative disputes.

For me the sentence or principle that is: "Every (potentially fertile) sexual act must serve only reproduction as its main purpose! (Anything else would be a sin!)" is not an axiomatic premise, but merely a possible conclusion from different premises.

At the end of Feser’s remarks in his book The Last Superstition, a reason is finally given why sexuality is not allowed to use the genitals for purposes other than those for which they were allegedly intended. The brief justification is as follows: It is a matter of preserving the human species. 
Where certain natural functions concern only some minor aspect of human life, a frustration of nature’s purposes might be at worst a minor lapse in a virtue like prudence. But where they concern the maintenance of the species itself, and the material and spiritual well being of children, women, and men – as they do where sex is concerned – acting contrary to them cannot fail to be of serious moral significance. (Feser - Last Superstition)
Thus we get an argument, why the deliberate frustration of the reproductive purpose during the sexual act is intrinsically wrong so that with the issue of sex no exceptions are tolerated. This is the argument put forward by Aquinas himself:
Nor, in fact, should it be deemed a slight sin for a man to arrange for the emission of semen apart from the proper purpose of generating and bringing up children, on the argument that it is either a slight sin, or none at all, for a person to use a part of the body for a different use than that to which it is directed by nature (say, for instance, one chose to walk on his hands, or to use his feet for something usually done with the hands) because man' s good is not much opposed by such inordinate use. However, the inordinate emission of semen is incompatible with the natural good; namely, the preservation of the species. (Summa Contra Gentile XIV. 9)
Unfortunately, neither Feser nor Aquinas bothers to explain this in more detail, perhaps they suspect that they might get into an argumentative circle. Because one could ask now: Why is the preservation or maintenance of the species so important? And one would get the answer: Because sex is aimed at preserving the species. This, in turn, would provoke the next question: But why shouldn’t I undermine the possibility of reproduction during sex? Answer: Because the preservation of the species is important. The preservation of the species justifies sex and sex justifies the preservation of the species. One justifies the other and the other way around. As I said, such a justification would be the pure circle. In my opinion, it is indeed hidden in Aquinas reasoning:

A: Why am I not allowed to undermine procreation during fertile sex and why do I have to go for it?
B: Because procreation is important.
A: And why is procreation important?
B: Because sex is aimed at procreation.

A: Why is frustrating the function of sex wrong?
B: Because the function of sex is preserving the species.
A: Why is the preservation of the species so important?
B: Because the function of sex is preserving the species.

The extra reference to the preservation of the human species is therefore not particularly informative, it is rather circular. Elsewhere, however, Aquinas suspiciously says that his ethics and its principles are simply self-evident and therefore need no logical justification. As with Moses, then simply in the moral sphere, a "That is so, no objection" is created. But this rationale seems to me to be in the clutches of the Munchhausen trilemma. The dilemma is then simply covered with clever rationalizations. Simply to assume that the supposedly intuitive moral result in the field of sexuality is right and true means to ignore the real question. The question is: Why is this result right and true in itself?

Moreover, the natural lawyer considers the reproductive ability in sexual activity only for itself, isolated and as if it were a completely independent good "with absolute rights of its own" (Grisez) and not just one ability among many others that are subordinate to man as a whole. Peter Vardy also draws attention to this problem:
It may also be argued that Aquinas’ approach is not holistic – it fails to consider the human being as a psycho-physical unit. To separate, for instance, genitalia out as having a particular purpose on their own without considering the whole complexity of a person’s relationship to his or her body, psychology, sexuality in general, the ability of human beings as embodied persons to express and receive love and to come to their full humanity may be a diminution of human beings as people. We are not an accumulation of ‘bits’ – we are whole human persons and all moral judgements must take our complexity as human persons into account. (Vardy, Peter. The Puzzle of Ethics)
I will now let two defenders of lived-out homosexuality speak at length, who refer to the above Aquinas quote:
In response to the example of walking on one’s hands, Aquinas claims that “man’s good is not much opposed by such inordinate use”; homosexual acts, by contrast, undermine the great good of procreation. There are two problems with this response. The first is that it is by no means clear that procreation is the only legitimate good achieved in sex, or that it is morally necessary for every sexual act to aim at it. Heterosexual couples often have sex even when they don’t want children, don’t want more children, or can’t have children. Most people recognize that sex has other valuable purposes, including the expression of affection; the pursuit of mutual pleasure; and the building, replenishing, and celebrating of a special kind of intimacy. In order to maintain Aquinas’s position, one would have to contend either that those purposes are not genuine goods or that homosexual acts cannot achieve them. These contentions both seem false on their face, although I’ll address them at greater length when I discuss the “new” natural law theorists below. The second problem is that the failure to pursue a good—in this case, procreation—is not equivalent to undermining or attacking that good. Aquinas himself was a celibate monk, after all. As the utilitarian philosopher Jeremy Bentham sharply observed over 200 years ago, if gays should be burned at the stake for the failure to procreate, then “monks ought to be roasted alive by a slow fire.” The issue of celibacy aside, there are plenty of heterosexuals who procreate abundantly while also occasionally enjoying, say, mutual masturbation or oral sex to orgasm. Such persons can hardly be said to undermine the good of procreation any more than Aquinas himself did. (John Corvino – What’s Wrong with Homosexuality)
And:
In an early work (Summa contra gentiles 3.122) Saint Thomas had predicated his objection to homosexual activity not on animal sexuality but on an argument which many later theologians were to seize upon in regard both to contraception and” unnatural” sex acts-that semen and its ejaculation were intended by “nature” to produce children, and that any other use of them was” contrary to nature” and hence sinful, since the design of” nature” represented the will of God. Unlike later writers, however, Saint Thomas realized that this argument had fatal flaws. He himself raised the question of other” misuses” of “nature’s” design. Is it sinful for a man to walk on his hands, when” nature” has clearly designed the feet for this purpose? Or is it morally wrong to use the feet for something (e.g., pedaling an organ) which the hands ordinarily do? To obviate this difficulty, he shifted ground and tacitly recognized that it was not the misuse of the organs involved which comprised the sin but the fact that through the act in question the propagation of the human species was impeded. This line of reasoning was of course based on an ethical premise-that the physical increase of the human species constitutes a major moral good which bore no relation to any New Testament or early Christian authority and which had been specifically rejected by Saint Augustine. Moreover, it contradicted Aquinas’s own teachings. Nocturnal emissions” impede” the increase of the human race in precisely the same way as homosexuality-i.e., by expending semen to no procreative purpose-and yet Aquinas not only considered them inherently sinless but the result of “natural” causes. And voluntary virginity, which Aquinas and others considered the crowning Christian virtue (Summa theologiae 2a.2ae.151, 152), so clearly operated to the detriment of the species in this regard that he very specifically argued in its defense that individual humans are not obliged to contribute to the increase or preservation of the species through procreation; it is only the race as a whole which is so obligated. Because of this, Aquinas found it necessary to shift ground again in formulating theological opposition to sexual nonconformity in his major and most influential moral treatise, the Summa theologiae. There are three substantive comments on homosexuality in the Summa. In the last and best known of these Aquinas discusses under two headings (I) whether “vices against nature” constitute a species of lust (he concludes they do) and (2) whether they are the most sinful species of lust (they are). “Vices against nature” include masturbation, intercourse with animals, homosexual intercourse, and nonprocreative heterosexual coitus. Although nature is defined elsewhere in the Summa in many different, sometimes conflicting ways, ranging from ”the order of creation” to “the principle of intrinsic motion,” no definition is provided here for the “nature” these sins are against, and all common conceptions of “nature” are missing from or excluded by the particulars of the discussion. “Animal” sexuality is opposed to the “natural” at one point, and no other sense of “nature” suggested would apply any more to homosexual acts than to procreative extramarital sexuality. Although at one point he does remark that the potentially procreative types of lust discussed earlier under” fornication” and “adultery”, do not ”violate human nature,” this is directly contradicted by his assertion in the treatment of “fornication” that “it is against human nature to engage in promiscuous intercourse.” Indeed, as he subsequently admits, not only are all sexual sins “unnatural,” but all sins of any sort are ”unnatural.” The ”natural” in this section is in fact simply the ”moral”; and it seems circular, to say the least, to argue that homosexual acts are immoral because they are immoral. (John Boswell – Christianity, Social Tolerance and Homosexuality)
The individual human being has no duty to conceive children, but humanity or mankind does. This is an absurd thesis by Aquinas, which is more than a paradox. For what is humanity other than the sum of the individual human beings? If humanity were more than that sum, it would call every individual to make children.

Aquinas brings two examples of harmless misappropriation: Walking on the hands or using the legs and feet as arm and hand replacements. According to Aquinas, these are small sins or no sins at all. Feser then gives in his book The Last Superstition an example, not directly for a misappropriation (if there is any difference at all), but for an action directed against the realization of a purpose of the body, namely when one tries to clean one’s ears with cotton swabs, although the ears clean themselves. The ears do this by pushing the earwax, which catches dust or bacteria, for example, and absorbs dead skin cells, outwards to the outer ear, while the cotton swabs push it back inside (contrary to the natural direction): The substance is constantly formed in the ear and then by fine hairs in the ear canal slowly pushed towards the ear outlet. In addition, earwax is a mixture of fats that keeps the pH value of the skin low, which makes germs less able to survive. For Feser the use of cotton swabs is nevertheless only a small virtue sin, a tiny offense. But this sin is not really that small. One expert warns:
If you use cotton swabs inside your ear, you run the risk of getting seriously injured. Since the auditory canal is quite narrow and the skin in it is very sensitive, the swab can easily cause bleeding. In rare cases, it comes to drumhead tears or cuts. Some injuries even require surgery. (some German web source)
In some cases, the skin is also temporarily no longer protected by the removal of the wax and therefore quickly dries out. Inflammations of the auditory canal, which can become chronic, and eardrum injuries can eventually develop into hearing loss, including deafness. So, if I clean my ears with cotton swabs, there is a risk of hearing loss and destroying my auditory system, even if it seems very low, according to doctors. However, according to Feser, the use of cotton swabs is harmless, even though a great danger is present, and even though the ear cleans itself in a teleological process. Does Feser really see the danger of destroying an important sense organ as just a small sin? If I scratch my eye with a pen, would it be just as insignificant?

Be that as it may. Purposes of bodily functions can therefore indeed be frustrated without great sin, or some bodily organs can also be misappropriated if the general human good is not injured (completely sinless is a teleological deviation if it serves a legitimate higher purpose). Aquinas also mentions some human goods such as self-preservation, maintenance of the human species, social life, and sainthood. These also seem to be in a kind of hierarchy that urgently demands justification. Furthermore, I do not see any fundamental difference between self-preservation and the preservation of the species. 

A side note to the cotton swabs: Anyone who cleans their ears a little with cotton swabs and knows that experts advise against it, does not want to work contrary to the natural cleaning process of the ears, but rather wants to help them on their way. The statements of the experts are not taken very seriously then. Who would really have the intention to damage a physical process with the cotton swabs (contrary to), and who would have any other purpose than the cleaning one (like sensual ear tickle) with the help of the swabs?

Basically, the example of Edward Feser with the cotton swabs for removing earwax is badly chosen as a minor offense (it is either no offense at all or just a real offense like any other one too).

In my opinion, however, the purpose of the sexual bodily function was not to be frustrated (which means the same to me here: the sexual parts of the body should not be misused) because two premises (but not in a logical sense, more in an emotional sense) that were considered very important in ancient times stood in the way:

1) One should preserve the human species, i.e. reproduce. The sexual act acquires its moral weight by reference to the preservation of the human species, which is supposed to constitute an important human good. (Sometimes one has the impression that the idea of the sexual act with ejaculation in the vagina is equated with the idea of the preservation of mankind, sometimes again no equation takes place in order to have a moral justification for the correctly proceeding sexual act. The suspicion of sophistry is definitely given.)

2) The male sperm potentially contains the complete living "individual body" of man. So the animal soul/form of man is already fully contained in the semen. This means that the semen contains the form and movement impulse of the male person.

According to Anthony Kenny, it was believed in older times that the individual body already existed before conception in the form of the fatherly semen. Aristotle thought very similarly, and why should this be only a small insignificant detail for Aquinas and the first Thomists who adopted this train of thought? The idea goes back more than two thousand years. It has thus perhaps somehow become emotionally entrenched in us and still subliminally suggests that man’s semen is something quite sacred. We all know the matching Monty Python song:


Let's listen to Anthony Kenny on this subject:
But in addition to those who thought that the individual soul existed before conception, there have been those who thought that the individual body existed before conception, in the shape of the father’s semen. Onan, in Genesis, spilt his seed on the ground; in Jewish tradition this was seen not only as a form of sexual pollution, but an offence against life. Aquinas, in the Summa Contra Gentiles, in a chapter on “the disordered emission of semen” treats both masturbation and contraception as a crime against humanity, second only to homicide. Such a view is natural in the context of a biological belief that only the male gamete provides the active element in conception, so that the sperm is an early stage of the very same individual as eventually comes to birth. Masturbation is then the same kind of thing, on a minor scale, as the exposure of an infant. (Anthony Kenny – Life stories)
And:
Aquinas is often invoked in contemporary discussions of the morality of contraception and abortion. In fact, he had very little to say on either topic. Contraception is discussed, along with masturbation, in a question in the Summa contra Gentiles concerning ‘the disordered emission of semen’. Aquinas maintains that this is a crime against humanity, second only to homicide. This claim rests on the belief that only the male provides the active element in conception, so that the sperm has an individual history continuous with the embryo, the fetus, and the infant. In fact, of course, male and female gametes contribute equally to the genetic constitution of the eventual human being. An embryo, unlike the father’s sperm or semen, is the same individual organism as an infant at birth. For Aquinas, the emission of semen in circumstances unsuitable for conception was the same kind of thing, on a minor scale of course, as the exposure or starvation of an individual infant. That is why he thought masturbation a poor man’s version of homicide. (Anthony Kenny – Medieval Philosophy)
So there is no complete biological and individual preform of the human being in male sperm that could claim rights over others. This was an old belief, inspired by Aristotle and even older Greek thinkers, which the Thomists adopted without scrutiny.

Premises 1 and 2 together, which in reality are not logical premises, in my opinion, historically led to the ethically weighty conclusion: Any fertile sexual act must serve only reproduction as its highest purpose! Anything else would be a sin! If you drop both, however, the sexuality that prevents reproduction is quite obviously on the same low moral level as walking on one’s hands or cleaning one’s ears with cotton swabs, which is harmless for Feser. Sex for mere fun could, therefore, be a morally very unimportant thing. There is nothing left to put a heavy moral weight on the act of sexuality, setting aside theological aspects such as the transmission of sin. 

What happens if one only maintains the first premise, since the second is already scientifically refuted. The first premise is so unspecific and general that it does not logically follow that any (potentially fertile) sexual action must be primarily aimed at reproduction. The maintenance of the species is also subject to other factors such as the environment and politics (overpopulation, for example, is an important reproductive factor for Aristotle); and factors such as personal maturity (as explained above), health (too old couples must even abort for Aristotle) and material resources must also be considered. The motto of natural law, to conceive as many children as possible, completely ignores the ideas of ethics of responsibility.

According to Aristotle, man is a political and social being, i.e. a being that needs the state. Since man is so by nature, can it not be said that the requirements of the state determine the final cause and the interpretation of the principle of totality for all human actions? In some sense, Aristotle saw it that way, for he has set the polis as a moral standard. Hegel then took this view to extremes.

The only important thing is that, as a rule, all humans are expected to reproduce sooner or later. And if it is intended concretely, then only under circumstances that are conducive to the preservation of humanity. The following would then not be allowed: self-castration, total avoidance of child production in long-lasting, well-off, and fruitful marriages, gay marriage, and the priestly vow of virginity, whereby every woman is avoided and the sexual organ is left atrophied.

Independent of all this, the premise itself is problematic. Antinatalism provides some convincing arguments (cruelty/harshness of the world, politically difficult times, gloomy prospects for the future), making the first premise obsolete. And what's more, the moral importance of the preservation of mankind is not proven. It must under no circumstances be merely asserted, and it must under no circumstances be only theologically justified. Particular attention must be paid to any possible circular reasoning. The preservation of mankind is an amoral affair. There are people who very much approve of the preservation of mankind, and there are also people who promote the opposite, and finally, there are the indifferent people as well. There are only moral obligations against already existing people. Catholics were perhaps overly influenced in this respect by the ancient Hebrews. Yahweh's Old Testament commandment to multiply may play a big role. Here is an example by Rabbi Eleazar ben Azariah (ca. A.D. 100):
Whoever refuses to marry violates the commandment to increase and multiply, and must be looked upon as a murderer who lowers the number of beings created in the image of God. (quoted from Uta Ranke-Heinemann - Putting Away Childish Things)
Rabbi Eliezer (around A.D. 90) made a similar comment:
Whoever does not attend to propagation is like one who sheds blood. (quoted from Uta Ranke-Heinemann - Putting Away Childish Things)
The old Jews thus had the thought that if you don’t reproduce, it would be like murder.

One can now compare two different types of preservation: Preservation of mankind (in the sense of constantly producing offspring) versus preservation of one’s own auditory sense, a very important sense associated with reason. For me personally, the latter is clearly more important and more significant, this is my own individual opinion. Feser won’t be able to change my mind. So, if I accepted natural law, cleaning the ears with cotton swabs would be the bigger sin (if at all) for me than a coitus interruptus (which still has some sperm released into the vagina). And for me, coitus interruptus is basically like any other contraceptive that only differs in pregnancy likelihood.

Moreover, the natural lawyer relativizes his own basic premise two. Because nobody has the moral obligation to get involved in the natural process whose natural end includes possible fertilization of an egg cell. A (relativizing) choice is given. However, it will hardly exist in relation to health, sociability, and holiness. In addition, the natural lawyer allows the removal of diseased generative organs for the benefit of the whole body (an example is given further below):
Natural law theorists do not object to the removal of a diseased uterus and ovaries, for example, even though such surgery impedes the functioning of a woman’s reproductive organs by removing them. (Weithman, Paul J.: Natural Law, Morality, and Sexual Complementarity)
This principle of totality would be something like this: Each imperfect part is dedicated solely to the perfect (de facto teleological) whole.

Why then should contraceptive sexual intercourse not serve the good of the whole in some form, however trivial (sexual activity is now regarded as a basic need and the nonfulfilment of this need as a lack of quality in life)? I would have to say that this non-reproductive sexual intercourse is not beneficial to the good of the whole and that it even damages that good. How would do I do that? To what extent would it be damaged? If anyone objected: Why do you need to make use of this principle for sex, namely through safe sex? My answer would be: Pregnancy and a baby would be disadvantageous for the totality of myself, contraceptive sex would be to the full advantage of that totality (at least at the moment).

For the (common) good of the whole (or the overall benefit or the principle of totality) of man, I can treat animals (e.g. drinking cow’s milk, which is intended only for calves and therefore, in the opinion of some, is unhealthy for man, or killing whales or other animals threatened with extinction in order to obtain sexual enhancers from them) and artifacts (e.g. using a large book as a pedestal, or burning books to produce heat or smoke as a signal, or smashing dishes on the occasion of a certain traditional wedding celebration) improperly and contrary to their telos. Why shouldn’t this apply to all human abilities and faculties? Alan Donagan also asks himself this question:
If St. Thomas considers that rational beings have the right to interfere with and even destroy sub-rational natural things for their own purposes, why should he think it wrong per se for them to interfere, for their own purposes, with the natural activities in which they engage? (Donagan, Alan: THE SCHOLASTIC THEORY OF MORAL LAW IN THE MODERN WORLD. In: Aquinas – A Collection of Critical Essays edited 1969 by Anthony Kenny)
I am allowed to give away single body parts or temporarily block and pervert them for the purpose of my physical and mental health. Why does sexuality occupy a special position in all these possibilities? That being slightly different (from the perspective of natural lawyers) in sexuality (ejaculation outside the vagina) is always a big sin and impossible a small one or none at all is simply a blank presumptuous assertion.

If I somehow feel like I’ve eaten something bad, then I’m allowed to induce artificial vomiting. If I sleep with a woman unprotected but have at the onset of orgasm the dark feeling that possible fertilization would not lead to anything good (perhaps in a sense that concerns mental health, which is also very important), why shouldn’t I use a contraceptive method like coitus interruptus?

If an organ is ill and endangers the whole organism, as in the case of testicular cancer, there is clearly something "evil" that needs to be eliminated. Amputation (if modern medical technology sees no alternatives yet) would be the only way to heal quickly. An organ is sacrificed here to the good of the whole. If, on the other hand, one is very sexually frustrated and this frustration (since Sigmund Freud really no longer a trifle!) burdens the whole psyche (and thus in hylomorphism the whole body), then there is also something "evil" that has to be eliminated as well. However, if I haven’t found my great love yet, in this case, a consensual casual and protected sex remains the only way to quickly stabilize and heal my very tense psyche. Here a possible pregnancy (so just an imaginary thing) is "sacrificed" to the good of the whole. An irreversible medical intervention and short-term use of contraceptives are thus opposed. Why should there be an unbridgeable ethical gap between them? Before one tries to answer, it would not be wrong to ask oneself another question: Why must the common good of man (the good of the whole, or the principle of totality) be interpreted in a Christian or religious way? Why can it not be understood under the aspect of good life as opposed to the aspect of ascetic pure survival? You should be aware: If it wasn't for 2,000 years of Christian bad-mouthing about sexuality, then perhaps there would be a somehow liberal interpretation of the principle of totality (regarding sexuality).

Someone who is not a Christian and makes some concessions to the ethics of natural law could at most say that sex with contraception is only qualitatively inferior or bad sex and that there's nothing morally evil about it. He might even say that bad sex is at least better than none at all.

The one who has to give a semen sample (by masturbating) so that his sperm quality can be checked because all his attempts at procreation have not yet been successful does everything for the sake of reproduction. Even under Thomistic criteria, there can be nothing wrong with this. For if I am allowed to frustrate a bodily function that serves self-preservation for the purpose of self-preservation, then I am also allowed to frustrate a sexual function that serves procreation for the purpose of procreation.

If I am allowed to wear glasses to improve my vision, I may also artificially transport my semen to an assisted fertilization device if biologically necessary to improve fertilization. This means that I do not necessarily have to ejaculate in the vagina. But is my sperm so valuable that I somehow have to get it quickly from that device into a vagina before it dries out completely? Can't I just bring new semen into the vagina at a later time, on another day or in another year?

If anyone thinks there are only hedonistic reasons for masturbation, one can mention an anecdote about Diogenes of Sinope. He was more concerned with the temporary elimination of what he saw as a probably annoying and unwelcome urge and need. Getting rid of an evil cannot be matched with hedonism as it is popularly understood. Here's the anecdote:
Behaving indecently in public, he wished it were as easy to banish hunger by rubbing the belly. (DIOGENES LAERTIUS - Lives of the Eminent Philosophers)
So Diogenes of Sinope masturbated in a crowded market also to convey a philosophical and political message in a provocative way. But you simply cannot condemn him, this famous and sometimes extreme Cynic philosopher. That would be silly and narrow-minded. Actually, he should be celebrated and, by the way, the Cynical philosophical school, to which he contributed substantially, probably influenced Jesus of Nazareth as well. For Cynical philosophy was very common in Galilee in the days of Jesus, so he must have picked up some of it.

Let’s look at a convincing example by Grisez "on the conduct of women engaged in lactation". By lactation, Grisez does not mean the ongoing and involuntary secretion of milk by the mammary glands, but the action of suckling an infant (Oxford Dictionary of English):
In many cases there is excess milk and it is pumped out of the breasts and thrown away. The infant may be fed artificially during a temporary separation from his mother while she continues regularly to empty her breasts artificially and to waste their product. No one condemns this conduct nor even demands that there be a serious cause to justify it. (Germain Grisez: Contraception and the New Natural Law – Chapter 2: INADEQUATE ARGUMENTS https://catholicebooks.wordpress.com/2014/07/25/online-text-contraception-and-the-natural-law-by-germain-grisez/)
And:
Yet lactation is the essential end of a very important natural faculty. And, like sex, it depends upon depositing a valuable glandular secretion in the appropriate natural receptacle. But mere convenience is a good enough reason for interfering in this process. Hence if contraception really is seriously wrong [which we figured out isn't the case] there must be some reason for its malice that has nothing to do with what these two cases have in common -i.e., preventing an important faculty from attaining its natural end. (Germain Grisez: Contraception and the New Natural Law)
Sexuality is the most important bastion of arch-conservatism against the liberal (dominant) tendencies in society. Conservatives want to defend this bastion, by all means, otherwise, they will lose the cultural battle forever. This is why they are so stubborn.

Moreover, natural law has many superstitious elements. There is often a superstition not to shake and shatter a particular order of natural things. The intention of the actor is usually ignored. One just doesn't want to have a false outward order of certain things in nature, so that one is not somehow persecuted by demons or charged with some God's personal wrath. In cultural history, a lot of things and acts were often perceived as a great insult to the many pagan deities and ancient ancestors still living somewhere in the spirit world. The afterlife felt strongly offended, as we believed when something was treated differently than expected by the spirit world (as we humans at least understood the supposed spiritual instructions for use).

The supposedly correct order of things, which according to the Church should be pedantically observed, seems in the Catholic order of values far above the good intention to achieve the true goal. A quotation by church critic Uta Ranke-Heinemann from her very readable book Eunuchs for the Kingdom of Heaven: Women, Sexuality, and the Catholic Church makes this clear:
Thomas's theory has implications today for artificial insemination, which was forbidden in 1987 by the Vatican Congregation for the Faith: "Homologous artificial insemination (italics mine) within marriage cannot be allowed." There is one exception, however: Semen can be obtained from intercourse by means of a condom, if this condom is perforated, so that the form of a natural act of generation remains intact, and no impermissible mode of contraception occurs. The conjugal act must take place as if it were leading to procreation, as if it were possible for conception to take place through the holes in condom (cf. Publik-Forum, May 29, 1987, p. 8). And only by this roundabout path, by an infertile conjugal act proceeding as if it were fertile, can fertility be helped along. The supposedly natural act has become the first commandment and it has kept that status even when its original goal, as prescribed by the Church, procreation, cannot be reached at all, and when obtaining semen through masturbation would be just as good a method, or a better one, because it is less complicated. But masturbation still ranks with the most serious, unnatural sins of contraception, even here when it is precisely being used to make conception possible. The standardized procedure has become more important than the goal, namely procreation. What is "natural" is determined by old traditions, and such traditions are carefully protected by old male celibates. (Uta Ranke-Heinemann - Eunuchs for the Kingdom of Heaven: Women, Sexuality, and the Catholic Church)
That is what underlies the whole, it is a superstition. And perhaps there is something else that is hidden even under this superstition. Perhaps something biological, so that one could say cautiously:
It is certainly true that many religions teach young people that premarital and extramarital sex are evils to be avoided. But the results of our studies suggest that the causal arrow may go in the opposite direction as well. Not only can religion shape people’s sexuality, but people’s [biologically anchored] sexual strategies can also shape their religiosity. (Douglas T. Kenrick – Sex, Murder, and the Meaning of Life)
Jonathan Haidt deals with this evolutionary-psychological aspect of conservatism in his highly recommended book The righteous mind: why good people are divided by politics and religion.

Robert Kurzban also draws attention to this: 
The point is that in a species in which individuals can constrain others’ behavior with rules, you’d expect evolution to act to cause members of the species to favor rules that serve their reproductive interests even if they don’t know this is why they’re endorsing these rules. 
[...]
From an evolutionary point of view, sex is fundamental, and if there were going to be some area in which people wanted to set the rules, this would probably be among the first. 
[...] 
Organisms are better or worse off, in terms of reproductive success, depending on the rules of the mating game. Suppose that humans, like some of our primate relatives, are, potentially at least, somewhat polygynous, with the “best” males getting more than one mate, meaning some males will be
left with none. Low-quality males would have a deep, abiding, even crucial interest in rules that force everyone into monogamy—in such a world, the low-quality males do a lot better. 
[...] 
The key is that some people win and some people lose in the evolutionary sense under different regimes. I think this explains the large differences in views on sexual behavior. 
[...] 
My main purpose here has been to sketch what an argument might look like for explaining why people are inclined to constrain other people’s behavior. (Robert Kurzban - Why everyone (else) is a hypocrite: evolution and the modular mind)
Aside from everything said, I can think of a contraceptive that theoretically allows fertilization, i.e. is not contrary to nature, but statistically speaking, makes it 100 times less likely than winning a big lottery. This means that a single sperm would theoretically have a chance, but in reality, achieving its goal would be almost impossible. Such thought games lead natural law ad absurdum.
 
The penultimate word of this passage shall belong to Paul J. Weithman:
[It] is hard to see […] why facts about the natural functions of the reproductive organs are even morally relevant, let alone morally decisive. To suppose they are morally decisive is to suppose that there can be cases in which the intentions of agents are irrelevant to the moral worth of an act. It is to repose the moral worth of those acts in their physical properties. (Weithman, Paul J. - Natural Law, Morality, and Sexual Complementarity)
In the end, you have to say:
[T]he intention alone decides on the worth or unworth of the deed, which is why the same deed, according to its intention, can be reprehensible or praiseworthy. (ARTHUR SCHOPENHAUER - The Two Fundamental Problems of Ethics)
III.1) Scattered remarks and additions, sometimes more, sometimes less convincing

Problematic Aristotelian teleology

The Thomistic ethics of natural law is based above all on the Aristotelian idea that all things in nature act and occur according to final causes. Here is the first problem:
The Thomistic-Aristotelian conception of metaphysics is defective for a number of reasons. To begin with, Aristotelian metaphysics is rooted in a particular scientific conception of the world; namely, one in which the categories of teleological biology are primary. Once science departs from that model and embraces another such as Newtonian mechanism or even indeterminism, the metaphysics has become anachronistic. Reintroducing the teleology becomes a form of metaphysical slight-of-hand, in practice a form of obsessive natural theology condemned to potentially endless embarrassment, and intellectually a transparent anthropomorphic projection. (CAPALDI, NICHOLAS: USING NATURAL LAW TO GUIDE PUBLIC MORALITY - The Blind Leading the Deaf)
In addition, it is neither a posteriori nor a priori possible to recognize the ultimate and definitive purpose of a given thing, i.e. the purpose can neither be derived empirically from experience nor logically from pure concepts. Every assumption of a certain purpose always remains hypothetical. It always remains open whether there is not a completely different purpose or an additional one or none at all or even a superordinate one, which labels the first assumed one as a mere means. 

Possible highest goals of an organism shall be listed here: reproduction, self-preservation, increase of power, dying, subordinating oneself to a certain ecosystem, becoming part of an all-encompassing cosmic process, serving as food and so forth. The problem of teleological hierarchies is described here (as I said, you have to know the supreme purpose, otherwise you don't really understand the subordinate ones):
Granted that we can explain the function of some part or activity by reference to the preservation of the whole animal, does it make sense to speak of the function of the animal as a whole? Does it and its life serve a purpose? ‘What is a sheep-dog for?’ can be answered — by reference to the shepherd’s needs and desires. But ‘What is a dog for?’ sounds odd, as odd as ‘What is a star for?’ Aristotle has two moves at his disposal. First, by insisting that the individual dog is a member of the species dog he provides something beyond the individual that the individual life does help to preserve. The point of a dog’s life is to maintain the species, by living a canine life and bringing on a new generation. (But now: What is the species itself for?) (J. L. Ackrill – Aristotle the Philosopher)
Ackrill carries it on: 
Special problems about teleology arise when we move from artefacts to natural organisms. It is easy to say not only what the function and purpose of each part of an axe is, but also what the function and purpose of the axe as a whole is. Before the craftsman set out to design and make an axe, he knew exactly what he wanted to do with it, the 'good' he was seeking to bring about. Now with an animal we can certainly explain how some organ serves to keep it alive, given the environment in which it lives and the other kinds of animal it has to contend with. Given such facts, it is useful to the elephant to have a trunk, and we can explain the purposes it serves in the elephant's life. But can we say what the function and purpose of the elephant as a whole is? A part 'serves a purpose' in helping the animal to survive; but what purpose is served by the existence and survival of the animal itself? What is the elephant for? What are rats for? It used to be suggested that all other animals, as well as plants, are there for the benefit of man -- leaving to be answered only the question why God made man. A modern idea would be that particular species of animal play a role in the wider ecological system. Rats keep down some other species which would, if uncontrolled, take over; they help maintain the 'balance of nature'. But then why should these particular species exist at all, and why is this particular balance of nature a good one? Similar questions await Aristotle. He holds that the function of an elephant is to produce another elephant: it is the survival of the species, not of the individual, that is nature's end. But what good is served by the existence of the elephant species, and by the existence of the other species there are? Are they in any way better than other species that there might have been? If not, we may be able to see how natural processes work for the preservation of the universal status quo, but we shall hardly be entitled to say that nature works for the good, that it produces the best of all possible worlds. (J. L. Ackrill – Aristotle the Philosopher)
Frederick C. Beiser sums up Kant's view on teleology:
Kant had argued that the concept of a final cause should have only a regulative status in the explanation of nature, i.e., we should treat nature as if it acts for ends because this brings systematic order to our explanation of nature; but we have no right to assume that nature really does act for ends. […] Final causes are crucial for our anthropomorphic and anthropocentric way of explaining things; but we have no insight into the purposes of nature itself. (Frederick C. Beiser – Late German Idealism)
According to Kant, to explain nature with purposes in an objective sense would be an overbearing assumption. 

The Bible (and Aquinas himself says that he is a man of only one book -, and that is, of course, the Bible) also virtually denies the possibility of fathoming the essence of things:
No one can comprehend what goes on under the sun. Despite all their efforts to search it out, no one can discover its meaning. Even if the wise claim they know, they cannot really comprehend it. (Ecclesiastes 8:17)
And:
No one can comprehend the height of heaven, the depth of the earth, or all that goes on in the king’s mind! (Proverbs 25:3)
Aristotle and Aquinas both assume that natural things act to achieve what is best. All bodies of nature always strive for the best and effect the best ("of possibilities"). Fritz Mauthner criticizes this optimism-laden value that has been smuggled in:
Already [Aristotle] has the maxim that nature always makes the best of possibilities, in fact that optimism which Voltaire regarded as ridiculous and Schopenhauer as ruthless. (Fritz Mauthner – Aristotle https://archive.org/details/aristotle00maut/page/10)
So according to Aristotle and Aquinas, all things strive for their own good and probably for the good of the universe. In order to be able to say what is good or best for a piece of iron, for example, one would somehow have to know in advance what is best for all (metallic) inorganic things in a solid-state. But how should I know what the best thing is for a little piece of iron under any circumstances? I could say that it always makes the best of itself in any situation, no matter what it’s doing. However, iron is iron qua iron and therefore does everything it does in its specific way because it is iron. Here, there is a pure necessity (teleological or not), which in itself is first of all value-free.

Be that as it may, according to Aristotle the concept of the natural place now comes into play. The doctrine of the natural place, of the locus naturalis, also belongs to the idea of final causation. When one throws a piece of iron up, it falls back to the ground because it has its natural place on earth. To be there belongs to the essence of that metal. Therefore, the iron wants to remain on the earth, its natural residence. To stay at this place is good or best for that iron in terms of value. Now, of course, Aristotle could not know all this, he could not derive it inductively or logically with absolute certainty. He had only suspected it from his speculative and ultimately false cosmology so that the idea of a natural place in this form is no longer tenable. This famous appeal to a teleological principle at a basic theoretical level in natural science has been discredited in a way parallel to phlogiston and ether by modern scientists.

Concrete purposes result only from a dogmatic overall explanation of the world, as the example of Aristotle's natural place has shown. Without such an explanation one always floats in the unknown, which purposes could exactly exist.

In the following Fritz Mauthner criticizes Aristotle in general with regard to teleology:
Aristotle created teleology in its coarsest form, and rather prides himself on having sought for traces of design everywhere. At the same time, he never laid a general foundation for his conception of design, but borrowed it, without examination, from common speech. We certainly owe countless suggestions and beautiful observations to the teleological view of nature: only, in such cases, the notion of design invariably supplies merely a stimulating question and not a satisfactory answer. Aristotle, however, with a childlike confidence already sees the answer in the question. He always sets his mind at rest too soon. His often repeated assertion that nature does nothing in vain seems to me to contain the pith of his erroneous natural philosophy. Aristotle thinks that he knows something where no other man has any knowledge at all. The assertion only sounds more impressive, but is quite as unverified and unverifiable as the exactly equivalent dictum: that nature always pursues an end. The whole theory is drawn from the notion of design as it is found in current speech. All the monstrosities of later teleology are thus already to be found in Aristotle. [...] Already he has the contemptible doctrine that plants exist on earth for the sake of the brutes, and the brutes for the sake of men. (Fritz Mauthner – Aristotle https://archive.org/details/aristotle00maut/page/10)
The anthropomorphism behind teleology is well described by David Lewes:
The spontaneous tendency to invoke a Final Cause in explanation of every difficulty is characteristic of metaphysical philosophy. It arises from a general tendency towards the impersonation of abstractions which is visible throughout History. We animate Nature with intentions like our own. We derive our ideas of Cause, and Force, from our own experience of effort; and the changes we observe are interpreted as similar in origin to the changes we effect. This leads to the Fetichism of savages and children; to the Polytheism of more advanced intelligence; and, by a gradual refinement in abstraction, to the Metaphysics and Transcendental Physics of later days. We first impersonate the causes as Deities; we next eliminate more and more of the personal elements, leaving only abstract Entities; we finally reduce these Entities to Forces, as the general expression of Properties or Relations; e. g., the Force of gravity is only the abstract expression of the fundamental relation which matter universally manifests. All matter is heavy; all masses attract other masses; this property is as universal and fundamental as that of impenetrability; we abstract it as gravitation or attraction. In this gradation the Will first disappears; next the independent Existence; leaving finally, an abstract expression of observed order. In the final stage we recognize that what was assumed to be an independent something, regulating phenomena, moulding them according to its nature, is only an impersonation of the order in phenomena, the statement in abstract terms of the very facts themselves. Thus, observing the facts of organic growth and development, physiologists have attributed them to the agency of a Plastic Force (vis formativa, Bildungstrieb), which moulds the heterogeneous materials into definite shapes. If, however, we seriously consider what this Plastic Force can be, apart from the phenomena, we are quickly led to perceive that it is only a name assigned to the observed order, a generalized expression of the facts, which has been personified, according to a well-known tendency. (David Lewes – Aristotle https://archive.org/details/aristotleinclud00lewegoog/page/n7)
The Thomist somehow seems to assume that what is effected is always also what is aimed at (the scholastic formula: the cause is aimed at the effect). This principle in connection with natural law is more than questionable and problematic.

Let's take the moon that orbits the earth. This movement of orbiting must have a cause, which is teleologically directed to exactly that orbiting of the moon. But it rather seems that the movement around the earth is only a result of the confrontation of at least two causes. Perhaps one cause of the moon is to let the moon move completely away from the earth in a straight line. Another cause then shows a pulling effect towards the earth. This means that the moon is teleologically moved to reach the earth. It would be very naïve to believe that the two causes, which are opposite in their direction, would have fraternally agreed on one end, namely the orbit of the earth. The movement of the moon is a completely teleology-free resulting movement, which is not aimed at by the two mentioned causes but emerges simply from a clash violently and necessarily. Perhaps almost all motions are resulting motions, so non-resulting motions are hard or impossible to find. Although this would not contradict the scholastic principle per se, one could not derive anything ethically relevant from it on the basis of the uncertainty presented.  

If we take self-movement into account, then one could say the following: Maybe the moon wants to be as far away from the earth. Or it wants to reach the surface of the earth and merge with it. Or it wants to get to the center of the earth and perhaps dissolve into a non-extended midpoint of that center. In all these strivings, there would be a counterforce. The orbiting of the moon around the earth would again be a resulting movement, which is not the goal of the moon itself.

In any case, Aquinas needs the already mentioned hidden aspect of value in order to see the first changer as God. For Aquinas, change is the realization of a meaningful form. With another concept of natural processes, even an evil demon could preside over the value-free, even worthless changes.

If value-based teleology is exaggerated, i.e. if the efficient causes and the final causes are not kept strictly apart so that both including the aspect of value are even united into one kind of cause, then this would have fatal consequences for natural law. It would then simply be dismissed as an ethical theory. After all, everything in the world would happen as it should, and for the best. One could simply put, not do anything wrong anymore and not talk about mistakes, malformations, or abnormalities in nature (this thought is basically given in the concept of providence). If the efficient causes are completely absorbed into final causes, so that we basically only have final causes, then natural law will indeed get into difficulties. Essential or accidental chains of efficient causes would become pure chains of final causes. And Aristotelian final causes are always directed at the good in terms of value, which good can then no longer be undermined by completely neutral causes because these do not exist anymore as such. So when all efficient causes are at the same time final causes, because their effect is precisely what is being aimed at, abnormalities would be included in these aims. In other words: If teleological determinations are completely exhaustive in the world, then there is obviously nothing inexpedient anymore. The concept of teleology would, by the way, also lose its meaning, since it can no longer be differentiated from its contrary.

The Thomist says that the universe in its entirety also has an Aristotelian cause. But if the universe has an efficient cause, then it must also have a final cause, a formal cause, and a material cause. So if there were an overall final cause of the universe, everything, including man’s intentions from the outset, would be unconsciously included in a great nexus of subservient final causes. Everything above man’s head would be predestined for all the future; and through his consciousness, as through a mere medium, the great chains of realization of this teleological predetermination would have to take place inevitably.

Nietzsche cannot do anything with a world of purposes:
Let us beware of thinking that the world is a living being. Where would it stretch? What would it feed on? How could it grow and procreate? After all, we know roughly what the organic is; are we then supposed to reinterpret what is inexpressibly derivative, late, rare, accidental, which we perceive only on the crust of the earth, as something essential, common, and eternal, as those people do who call the universe an organism? This nauseates me. Let us beware even of believing that the universe is a machine; it is certainly not constructed to one end, and the word ‘machine’ pays it far too high an honour. Let us beware of assuming in general and everywhere anything as elegant as the cyclical movements of our neighbouring stars; even a glance at the Milky Way raises doubts whether there are not much coarser and more contradictory movements there, as well as stars with eternally linear paths, etc. The astral order in which we live is an exception; this order and the considerable duration that is conditioned by it have again made possible the exception of exceptions: the development of the organic. The total character of the world, by contrast, is for all eternity chaos, not in the sense of a lack of necessity but of a lack of order, organization, form, beauty, wisdom, and whatever else our aesthetic anthropomorphisms are called. Judged from the vantage point of our reason, the unsuccessful attempts are by far the rule; the exceptions are not the secret aim, and the whole musical mechanism repeats eternally its tune, which must never be called a melody – and ultimately even the phrase ‘unsuccessful attempt’ is already an anthropomorphism bearing a reproach. But how could we reproach or praise the universe! Let us beware of attributing to it heartlessness or unreason or their opposites: it is neither perfect, nor beautiful, nor noble, nor does it want to become any of these things; in no way does it strive to imitate man! In no way do our aesthetic and moral judgements apply to it! It also has no drive to self-preservation or any other drives; nor does it observe any laws. Let us beware of saying that there are laws in nature. There are only necessities: there is no one who commands, no one who obeys, no one who transgresses. Once you know that there are no purposes, you also know that there is no accident; for only against a world of purposes does the word ‘accident’ have a meaning. Let us beware of saying that death is opposed to life. The living is only a form of what is dead, and a very rare form. Let us beware of thinking that the world eternally creates new things. There are no eternally enduring substances; matter is as much of an error as the god of the Eleatics. But when will we be done with our caution and care? When will all these shadows of god no longer darken us? When will we have completely de-deified nature? When may we begin to naturalize humanity with a pure, newly discovered, newly redeemed nature? (Nietzsche, Friedrich. Nietzsche: The Gay Science)
The Thomist is unlikely to convince anyone of the benefits of teleology in the inorganic domain. People simply accept the characteristic chemical elements that exist, and they accept that in certain circumstances these elements act in their own way because they are what they are. Chemical elements simply have no (overarching) goals. And even if they existed, they could not be empirically identified. Anthony Kenny also sees problems:
Explain, if you like, the freezing of water by saying that water has a natural tendency to freeze; but why say that water is aiming at anything in freezing? Aquinas’ argument suggests that what is being aimed at is the freezing itself; but this amounts to saying that water freezes because it freezes. (Anthony Kenny - The Five Ways)
Aristotle says in principle nothing more than things do what they do because that is the sort of thing they do. And he only uses the word/concept final cause to express this. One could legitimately say that there is no particular explanation in that.

If life has arisen from chemical compounds, this will only confirm the non-teleological naturalistic view. But now associating life with teleology involves a mere tautology:
But to speak of the purposefulness of living beings is a hopeless tautology. Life is explained by expediency [final causality or purposefulness] and expediency [final causes] by life. This becomes quite palpable when one uses the seemingly scientific word organism for the common word life. A living being is called an organism because organs work together in it, no matter whether one only wants to understand the bigger tissue complexes, which perform a common service (?), or the microscopic cells according to the latest linguistic usage. The definition always boils down to the fact that the organs are the cause of life and that at the same time they have their purposeful cause [final cause] in the living whole. At least two of these confused three expressions are worthless because each of them can only be defined with the help of the other two. Life becomes comprehensible neither by purpose nor by the organic (organismal or organismic); purpose neither by life nor by the organic. And so on. (my own translation from German, Fritz Mauthner - Wörterbuch der Philosophie, dictionary of philosophy)
Teleological categories must also be treated more like negative variables or like a working hypothesis, which is provisional. There must also always be caution when using teleological categories:
An assessment of the world on the basis of final causes is only admissible to the extent that the efficient causes show a certain direction, as it were a point at which they will converge in the future. But the greatest caution is necessary when determining such points, because the door is open to error. (my own translation from German, Philipp Mainländer - Die Philosophie der Erlösung, The Philosophy of Redemption)
If one says that only teleology can provide a regular order in nature, one can counter that:
On the contrary, order is simply the manifestation of causality, and causality is a derivative, a logical corollary, of the Law of Identity. To exist is to exist as something, and to be something is to possess specific, determinate characteristics. In other words, every existing thing has identity: it is what it is and not something else. To say that something has determinate characteristics is to say that it has a limited nature, and these limits necessarily restrict its range of possible actions. (George H. Smith - Atheism)
And:
Order […] is simply entailed by the nature of existence itself. (George H. Smith - Atheism)
And also:
Once we accept the fact of existence, we must also accept the fact that things are what they are (identity), and that they behave as they do in virtue of what they are (causality). (George H. Smith - Atheism)
And:
... to say that the same thing acting on the same thing under the same conditions may yet produce a different effect, is to say that a thing need not be what it is. But this is in flat conflict with the Law of Identity. A thing, to be at all, must be something, and can only be what it is. To assert a causal connection between ‘a’ and V implies ‘a’ acts as it does because it is what it is; because, in fact, it is ‘a.’ So long therefore as it is ‘a,’ it must act thus; and to assert that it may act otherwise on a subsequent occasion is to assert that what is ‘a’ is something else than the ‘a’ which it is declared to be. (H.W.B. Joseph  An Introduction to Logic quoted from George H. Smith - Atheism) 
Further:
However, serious objections and difficulties remain: (i) we surely cannot accept Aristotle’s contention that everything that happens regularly is for something, that regularity proves purposiveness. In an animal we select from all the regularities those that contribute to the preservation of the animal, and say that they are for something, serve a purpose. Many other regularities seem to be simply law-governed chemical or physical processes that may serve no purpose. (J. L. Ackrill – Aristotle the Philosopher)
Frederick C. Beiser points to another problem behind internal teleology, namely:
[H]ow does the purpose inhere in the thing? There is not only the problem of how something future, having an ideal existence, can act in the present, but also the issue of how something universal and ideal can exist in something particular and real. (Frederick C. Beiser – Late German Idealism)
And again, Mauthner:
Moliere makes his Aristotelian ask: "Si la fin nous peut émouvoir par son être réel ou par son être intentionnel ?" His French expositors treat this as a madcap jest devoid of meaning. This it certainly is not. Moliere has nailed to the counter with one short, sharp blow the distinctive puzzleheadedness of the Aristotelian, "Are final causes something actual in themselves, or do they operate after the manner of human intentions?" (Fritz Mauthner – Aristotle https://archive.org/details/aristotle00maut/page/10)
Perhaps this is an interesting addition:
[I]t [should] be remembered once again that final causes for Aristotle have nothing to do with deliberate intentions, on the contrary: intentions are rather efficient causes for him. (Detel, Wolfgang. Aristoteles: Grundwissen Philosophie, Basic knowledge philosophy)
They are also not something real in themselves, since something future does not exist, like the oak tree in the case of an acorn, nor do they exist independently from God. So they do not exist in themselves according to Thomism. What else could they be? They're just a way of anthropomorphic talking, a strictly linguistic matter, nothing more. 

Teleology, values, and commandments

Mauthner says that Aristotle's
whole attitude towards nature is grounded on the arrogant assumption that nature is to be appraised in proportion to the service which it can render to man. That might be merely a commonplace. In thought or speech we never get out of the anthropomorphic groove. Aristotle alone contrives to set up a standard valuation on a still narrower and more restricted scale. He values the brutes in proportion to their resemblance to man. But then the male sex is his sole criterion, and woman appears to him as mutilated man. Then again, as the free-born Greek, he sets up another standard, and the slave appears to him as a slave from birth, made by nature of inferior value. Hence we are not astonished when we meet with "inferior numbers," "inferior veins," "inferior dimensions"; "before" is superior to "behind "; "above" is superior to "below." The criterion of value is the weak point of teleology. (Fritz Mauthner - Aristotle https://archive.org/details/aristotle00maut/page/10)
More:
It was not Aristotle’s belief in conceptions alone that was congenial to the Christian view of the world : still more congenial was the way in which he brought natural phenomena under notions of value. The Aristotelian conception of design is a conception of value, and goes very far beyond the natural conception of design which human speech in its anthropomorphic way usually attributes to nature. (Fritz Mauthner – Aristotle https://archive.org/details/aristotle00maut/page/10)
In its argumentation, natural law proceeds in three steps, all of which, without exception, are highly problematic: Setting a purpose, attaching value, and deriving a moral imperative. Each of these steps should be examined tirelessly and exhaustively. The Thomists want to equate these individual steps as quickly as possible and not see them differentiated, which makes the whole thing only extremely vague for a non-natural lawyer. Even the first step of accepting a specific purpose in nature is not an easy one. The move from purposes to comparative value judgments is even more problematic. Also because of what Schopenhauer has to say about values in general:
Every worth is a comparative quantity, and it stands moreover in a double relation: first, it is relative, in that it is for someone, and secondly, it is comparative, in that it is in comparison with something else according to which it is evaluated. Displaced from these two relations, the concept worth loses all sense and meaning. (ARTHUR SCHOPENHAUER The Two Fundamental Problems of Ethics)
And:
Every worth is the evaluation of a thing in comparison with another, thus a comparative concept and a relative one, and precisely this relativity makes up the essence of the concept worth. The Stoics (according to Diogenes Laertius, Book VII, ch. already taught correctly: ‘that worth is the remuneration or equivalent value for something fixed by an expert; just as it said that wheat is exchanged for barley plus a mule’. (ARTHUR SCHOPENHAUER - The Two Fundamental Problems of Ethics)
These relations are always created by a subject and exist only through this subjectivity. The quality standards of Aristotle, for example, are completely unreflected. He started from unquestioned, time-related anthropology, probably also from his proud feeling of being a free Greek man and thinker, and from social necessities in his polis (Aquinas in addition also considered the dark and anti-fun Christian values). So it's very likely the case that Aristotle (in effect, arbitrarily) designates as the aim of a process that stage which he wishes to count as independently good.

One could say, however, that the slightly deviant is the good, the discreet deviator (like Aristotle himself was one with regard to Plato or the religious custom of his polis) is to be taken as a yardstick of the good. For without deviations there was and is no progress and no improvements. In the end, the value assessment in one way or another is a purely subjective process, shaped by prevailing ideologies, religions, philosophies, the state of the sciences, in short, the spirit of the times, and shaped by the evolving nature of man and finally by the individual character, that is, the very special, coincidental peculiarity of a single person:
The individual human being in a certain moment of time is the measure of value. (my own translation from German, Fritz Mauthner - Wörterbuch der Philosophie, dictionary of philosophy)
If we agree with the essentialist philosophy of Aristotle, and if we also say that all individuals more or less instantiate their essence, this does not necessarily include the concept of value or worth. If, for example, a squirrel has strong deformations and shows large anomalies, i.e. it has only instantiated its essence to a small degree, this is nonetheless a value-neutral matter. A degree of instantiation does not logically imply any value. Values are something added later. Moreover, under the right circumstances, any deformation could prove to be extremely favorable and thus good (or maybe God wanted the squirrel that way, or the squirrel feels very good in everything it does).  

Nietzsche draws attention to a weakness of philosophers who always take the human being of their specific time as the absolute standard:
All philosophers have the common failing of starting out from man as he is now and thinking they can reach their goal through an analysis of him. They involuntarily think of "man" as an aeterna veritas [something everlastingly true], as something that remains constant in the midst of all flux, as a sure measure of things. Everything the philosopher has declared about man is, however, at bottom no more than a testimony as to the man of a very limited period of time. Lack of historical sense is the family failing of all philosophers; many, without being aware of it, even take the most recent manifestation of man, such as has arisen under the impress of certain religions, even certain political events, as the fixed form from which one has to start out. They will not learn that man has become, that the faculty of cognition has become; while some of them would have it that the whole world is spun out of this faculty of cognition. Now, everything essential in the development of mankind took place in primeval times, long before the four thousand years we more or less know about; during these years mankind may well not have altered very much. But the philosopher here sees "instincts" in man as he now is and assumes that these belong to the unalterable facts of mankind and to that extent could provide a key to the understanding of the world in general: the whole of teleology is constructed by speaking of the man of the last four millennia as of an eternal man towards whom all things in the world have had a natural relationship from the time he began. But everything has become: there are no eternal facts, just as there are no absolute truths. Consequently what is needed from now on is historical philosophizing, and with it the virtue of modesty. (Friedrich Nietzsche - Human All-Too-Human, A Book For Free Spirits)
The most problematic, however, is the leap from comparative value judgments to non-comparative ought judgments (there is no ought-er in the sense of better). One (all the people) cannot infer from a purpose which I (so I myself, as spirit-salamander) consider to be good and which I merely suspect in nature a commandment which is valid for all human beings because then one falls more than ever victim to the naturalistic fallacy. At most, one can construct maxims for oneself from the way one’s own body ticks in order to become happier. But then the "language" of the body is a consequentialist language and those maxims can only be recommended, not more because they may not be useful to others. My conclusion: In natural law, there is clearly false objectivity. Morality and law must be freed from false objectivity of abstract values or norms. In this way, protection is provided against dangerous ideologizations.

L.A. Rollins expresses his concerns:
As I’ve said, natural laws and natural rights are inventions intended to advance the interests of the inventors (whom I shall call “natural legislators”). What is often involved is an attempt to manipulate other people into behaving as desired by a natural legislator, by duping them into accepting the values of the natural legislator as the values of nature. Thus, the personal, subjective preferences of a natural legislator are passed off as the impersonal, objective requirements of nature. For example, Frederick D. Wilhelmsen writes that, “Natural law insists that pornography … is bad and that it is bad not just for me, but for everybody, and it equally insists that not only must I not invade my neighbor’s property but that he must not invade mine or anybody else’s.” In other words, Frederick Wilhelmsen insists that pornography is bad for everybody [Feser says, for example, that watching porn is not per se morally reprehensible], and he equally insists that no one must invade anybody else’s property. But in order to give his personal preferences greater authority, Wilhelmsen pretends that it is nature who is doing all the insisting. (L.A. Rollins – The Myth of Natural Rights)
Kai Nielsen expresses all this in his own way:
That many plain men infer “X is obligatory” from “God wills X, …. X is in accord with our essential human nature,” “X is of the true nature of Being” and the like, indicates they have tacitly, and perhaps even unconsciously ‘(as in a Peirceian acritical inference) assumed “What God wills is obligatory,” “What is in accord with our essential human nature is good,” etc. But here these hidden premises are themselves moral judgments; the “is” in the above sentences is not the “is” of identity, and all these statements may be denied without self-contradiction. (Nielsen, Kai: An Examination of the Thomistic Theory of Natural Moral Law. In: Kai Nielsen: God and the Grounding of Morality 1991 https://scholarship.law.nd.edu/cgi/viewcontent.cgi?article=1038&context=nd_naturallaw_forum)
And:
But for the sake of this discussion I will grant what I believe to be contrary to fact, namely, that “Necessary Being” and “final cause” have an intelligible use. Let us also assume for the sake of the discussion that sentences like “Nature is purposive,” “Man and nature have a final end,” and “Men were created to worship God” are true. My crucial point is: Even if these sentences really can be used to make genuine statements that are in fact true, no normative or moral statements can be derived from them. (Nielsen, Kai: An Examination of the Thomistic Theory of Natural Moral Law. In: Kai Nielsen: God and the Grounding of Morality 1991 https://scholarship.law.nd.edu/cgi/viewcontent.cgi?article=1038&context=nd_naturallaw_forum)
Finally:
Someone might still claim that I have not yet really met the Thomist or Aristotelian case. I have assumed a “metaphysical system” or a set of categories in which fact (including “metaphysical facts”) and value are distinct, but, it might be urged, the Thomistic-Aristotelian system is denying just that, for some facts and values at least. When it is claimed that nature is purposive, there is just this conflation of fact and value. I find this claim obscure almost to the point of unintelligibility. It is the obfuscation I was complaining about at the end of VI. There seems to be no intelligible job for these words here. We cannot do the usual things with them and we do not know what new things to do except that in some exceedingly obscure way they indicate that some claim is being made about a more secure foundation for our morality. But even if we can intelligibly indicate how the factvalue or evaluativedescriptive dichotomy does not apply to a statement like “Nature is purposive” and even if we can show that these “metaphysical realities” are at one and the same time facts and values, I still do not see how we can derive ordinary moral or evaluative conclusions from them. (Nielsen, Kai: An Examination of the Thomistic Theory of Natural Moral Law. In: Kai Nielsen: God and the Grounding of Morality 1991 https://scholarship.law.nd.edu/cgi/viewcontent.cgi?article=1038&context=nd_naturallaw_forum)
Aristotle apparently did not dogmatically establish the definition of the ethically good: 
It is rather with the good functioning of man, not with his functioning per se, that Aristotle identifies man's good.

[...]

In other words, the human good is not simply the same as certain functions; rather, the human good is the good performance of those functions. This means that Aristotle is not a pure naturalist in the practical sphere; he defines the human good not simply in terms of certain facts of human operations or activities, but rather in terms of the good performance of those activities. He goes on to say that such good performance means that the activities are carried on "in accordance with virtue," i.e. with excellence. (ALAN GEWIRTH – THE ONTOLOGICAL BASIS OF NATURAL LAW: A CRITIQUE AND AN ALTERNATIVE) 
Hence the rationality of an act seems to play a far greater role. It must be so because the rational soul controls the animal and vegetative functions of the body. Rationality and biological functionality do not necessarily always coincide with Aristotle. 

Nicholas Capaldi draws attention to the general problem of a connection between Aristotelian metaphysics and morality:
A [...] defect of the Thomistic-Aristotelian metaphysical system is that it transforms morality into an intellectual exercise, the application of theory to practice or morality as the reflective observance of rules or ideals. Emphasis is put upon having a correct and defensible theory rather than on how to act. The ideals too quickly turn into obsessions. Inevitably moral sensibility is inhibited or even eroded in favor of an elaborate casuistry. The object seems to be to observe a rule instead of behaving in a certain concrete manner. It achieves the appearance of stability at the price of imperviousness to change. When change can no longer be resisted it occurs as a revolution rather than as an evolution. Obsession with rigid deductive structures and a preoccupation with logical systematicity has been destructive of both historical understanding and rational criticism. (CAPALDI, NICHOLAS: USING NATURAL LAW TO GUIDE PUBLIC MORALITY - The Blind Leading the Deaf. In: THE DEATH OF METAPHYSICS; THE DEATH OF CULTURE - Epistemology, Metaphysics, and Morality. Edited 2005 by Mark J. Cherry)  
An alternative

For a convincing alternative ethical theory, T. M. Murray is already on the right track:
It seems that what is distinctive of human nature, or what is universal to all of us in distinction from animals, is not our particular biological capacities, but conscious freedom to direct our lives within the limits set for us. Conscious freedom, or the ability to act for reasons of our own making, appears to be the most common basis of our human nature. (Murray, T. M. - Thinking Straight About Being Gay)
In the history of philosophy, the development of a secular moral standard begins with Kant. Before Kant, one could say, there was no real, or better said, convincing secular and at the same time objective ethics. With the famous idea, that man must not be treated merely as a means, but also always as an end in itself, Kant achieved great success for the secular movement of the European Enlightenment. However, according to Gerold Prauss in his book Moral und Recht im Staat nach Kant und Hegel (Morality and Law in the State after Kant and Hegel), Kant does not explain why people are identified with ends in themselves. As Prauss says, we should stop dealing with morality only from the perspective of the actor, as Kant does. The notion of an end in itself becomes much more understandable with regard to morality or right when the objective perspective of action is taken into account, namely the perspective of the person being treated. Morality is, therefore, not a solipsistic matter. It is always interpersonal.

As I said at the beginning, ought expresses a will of another human. According to Prauss - and this is philosophically new - we can deal with people in three ways: only as a means of what would be the epitome of evil (hurting, lying, killing and not helping people in need who cannot help themselves); both as a means and as an end in itself, which should characterize the normal interaction between people (taking a taxi, having your hair cut, buying a book from a bookseller, signing a contract, having consensual sex with someone, etc.), and finally only as an end in itself, which could be considered if a person is no longer able to help him or herself, for example, due to an accident. One cannot then simply walk past this person in need of help completely indifferently and unconcernedly. This person has the ability to understand him- or herself as a free creator of ends, which in turn leads to the fact that he or she can make himself or herself an end in itself and thus automatically an end for all other persons. The injured person makes a demand on the person present who can no longer escape this demand without moving to moral sidelines. The demand does not have to be articulated. It only has to be assumed as a possibility or potential. The person present is thus de facto morally obliged to help. This assistance is then the morally good action required in this particular situation. When we do not help someone in need, we do not solely prove to be non-meritorious but we commit an evil, whatever our maxims might be. A person in need is a true end in itself. And ends in themselves are logically binding on any conscious will. 

Prauss establishes self-knowledge or self-recognition (ability to see oneself as an end in itself) and interpersonality as the cornerstones of ethics.

Each one of us humans is a potential knowing/conscious will or a knowing/conscious end in itself. And because everyone knows this about himself as well as about everyone else, he is then also a knowing/conscious demanding, a knowing/conscious claim against everyone else, for whom this must, therefore, become an obligation. For everyone who in this sense is a conscious end in itself wants to be treated as such by everyone else: at least also as an end in itself or even only as an end in itself, depending on whether he is capable or incapable of self-help.

That is, in a nutshell, rational agents (can) issue imperatives all the time. But only in certain interpersonal contexts do these imperatives give rise to objectively binding moral obligations.

So you can say at the end:
Once we have explicitly asked ourselves why we should do anything just because nature does it, or why we should aid nature in her purposes, we see that there is no reason why we should. Let nature look to her own purposes, if she has any. We will look to our. (Richard Robinson quoted in Donagan, Alan: THE SCHOLASTIC THEORY OF MORAL LAW IN THE MODERN WORLD. In: Aquinas – A Collection of Critical Essays edited 1969 by Anthony Kenny)
The problem of shaving

According to Thomists, nature wants me to have lots of babies. But nature also wants me to have long hair, so she wants me to become a mophead. Even if there is no reason to let all hair grow long, at least a complete shave of all hair should be prohibited by natural law because the hair on different parts of the body certainly has a teleological function. And where there are functions, there are faculties. And where a biological faculty exists, there is also the possibility of perverting that faculty.

The biologists say that eyebrows have a certain function, as do eyelashes, axillary hair, and pubic hair. So there is no doubt that each of these particular hair phenomena represents a biological faculty in its own right. Each individual specific strand of hair, together with the other strands of hair of the same type, forms a faculty. Hair as a whole like pubic hair often has a protective or signaling function.

Aristotle agrees: 
Man has the hairiest head of all the animals. […] [I]t is for giving protection, so that the hair may give shelter and protection from excesses of cold and heat. The human brain, being the biggest and the most fluid of all, needs the greatest amount of protection[.] ( Parts of Animals II.14.658b2)

Both eyebrows and eyelashes exist for the protection of the eyes. The eyebrows, like the eaves of a house, give protection from the fluids running down from the head; the eyelashes, like the palisades sometimes put up in front of an enclosure, are there to keep out things that might get in. (Parts of Animals II.15.658b14)
Consequently, shaving would be immoral. Maybe it's not so clear with the beard. I could say the following about that: The beard hair, for example, grows teleologically and the beard itself has a (teleological) function, whatever that function may contain. But it must have a function, because why should the hair grow at all (understood in an Aristotelian sense)? Then shaving the beard is a violation against natural law. For shaving prevents the hair from exercising its continuous function as a beard. I have thus deliberately brought a faculty (beard) into a process (shaving), at the end of which the goal of the faculty is undermined, at least temporarily. This may not be a great sin in the eyes of the natural lawyers, but it is nevertheless a violation. There are good reasons to understand the beard as a faculty for male and masculine display. Some (bit bizarre in places) remarks by Schopenhauer about the beard should not go unmentioned:
I imagine that the final cause of the beard is the fact that what is pathognomonic, and thus the rapid change in the features of the face which betrays every hidden movement of the mind, becomes visible mainly in the mouth and its vicinity. Therefore, to conceal this from the prying glance of an adversary as something that is often dangerous in negotiations or in sudden emergencies, nature (knowing that homo homini lupus) gave man the beard. Woman, on the other hand, could dispense with it, for with her dissimulation and self-control (contenance) are inborn. (Arthur Schopenhauer - The World as Will and Representation, Vol. 1)
Although according to Schopenhauer the beard has a causa finalis, he strongly advises shaving:
Even as an external symptom of the coarseness that is becoming rampant you see its constant companion – the long beard, this mark of sex in the middle of the face, saying that one prefers masculinity, which one shares with the animals, over humanity, by first being a male, mas, and only afterwards a human being. The shaving of beards in all highly educated ages and countries arose from the correct feeling of the opposite, by virtue of which one wants to be first of all a human being, in a sense a human being in the abstract, disregarding the animal sex difference. (Arthur Schopenhauer – Parerga and Paralipomena Short Philosophical Essays)
I found this article conveniently:
Pubic hair has a job to do – stop shaving and leave it alone - Shaving pubic hair only removes a cushion against friction, leaves microscopic open wounds and exposes you to infections (https://www.theguardian.com/commentisfree/2012/aug/07/pubic-hair-has-job-stop-shaving)
Now one should imagine that this sentence is stated by God. And then you could also describe the act of shaving very dramatically. That means the complete shaving of the pubic hair is a deeply perverting act against nature, it is the temporary destruction of a function, therefore very contrary to. Even Schopenhauer said almost correctly:
The final cause of the pubes in both sexes, and of the mons Veneris in the female, is that, even in the case of very slender subjects, the ossa pubis shall not be felt during copulation, for it might excite aversion. (Arthur Schopenhauer - The World as Will and Representation, Vol. 2)
If the Thomist doubts the existence of this function, which is assumed by many experts, he takes an extremely big moral risk in case of a shave.

"PROBLEMS WITH AN INTUITIONIST DEFENSE OF MORAL ABSOLUTES" (all quoted)
In principle, one could appeal to intuition to support an alleged moral absolute against the use of general anesthesia or any drug that renders one unconscious and therefore unable to know, will, or sense pain. In an analogy with Anscombe’s rejection of contraceptive sexual acts, one could begin by observing that there is nothing wrong with natural states of unconsciousness and dullness to pain: wake-sleep cycles are completely natural (akin to cycles of fertility and infertility), and certain activities naturally produce endorphins that offer some resistance to pain. However, intentionally and artificially inducing a state of complete unconsciousness and deadness to pain is contrary to nature. To continue with the analogy, we could assert that although the primary means of knowing the intrinsic wrongness of anesthesia is through intuition or mystical perception, we can also offer two kinds of supplementary considerations. First, such a prohibition would be generally useful insofar as consciousness- and pain-reducing drugs (including barbiturates and opioids) are widely abused, leading to addictions, ruined lives, and crime; thus, following our general prohibition on anesthetics would spare people significant misery. Second, we could assert that without an absolute prohibition against anesthesia, we are left without a moral compass: choosing to use barbiturates or opioids would then become a matter of attitudes and motives, as say, with cut flowers. Who could then say whether it is more legitimate to seek freedom from consciousness and pain during surgery than during a period of intense grief or even for recreation? Of course, such arguments are not made by natural law philosophers. In fact, the matter of anesthesia is rarely addressed, and when it is, it is typically ad dressed as a prudential matter. One of few natural law philosophers to address explicitly the matter of anesthesia (even if only in passing) is Professor Geach, Elizabeth Anscombe’s husband, and collaborator in the revival of virtue ethics. While discussing “ temperance ” in his book, The Virtues , Peter Geach writes,
If it were a duty to be mentally as much alert as possible for as long as possible, this might speak against any consumption of alcohol at all. But of course there is no such duty. … Aquinas has remarked that it is a precept of reason that the exercise of reason should be intermitted; in sleep, and again in the sex act, this is quite normally the case; and again, there is nothing immoral in taking sleeping-pills or having a doctor give you anaesthetics for an operation. ( Geach, 1977, p. 134)
We are nowhere told why the intermittency of fertility is not normative for those who would artificially induce periods of infertility, whereas the intermittency of consciousness is normative for those who would intentionally induce states of unconsciousness; nor why contraceptive acts are to be labeled counter-natural, whereas anesthetic acts simply and legitimately mimic nature’s own cycles. (DUBOIS, JAMES M .: Is Anesthesia Intrinsically Wrong? On Moral Absolutes and Natural Law Methodology. In: Christian Bioethics , 14(2), 206 – 216, 2008)
The problem with drinking alcohol

Alcohol consumption for amusement is a clear offense against natural law. Willingly expose oneself to poisoning, which leads to the fact that one only has silliness in mind or becomes very aggressive, cannot be brought into harmony with natural law. That would be something that many Catholics who like to drink alcohol would never understand (they would never ever want to acknowledge it either). The Thomist may perhaps point to the natural-law purpose of sociability, which is promoted by alcohol consumption. But this kind of sociability has little value because it only has a dull, seemingly happy, unbridled, and primitive quality. The drunk is only for a short moment excessively pleasing to each person present (the next day it looks different again), he is quickly tempted to embarrassment, looking for a wife with ridiculous approaches and everything that comes out of his mouth are just empty promises, bad jokes or stinging remarks. And just to put oneself in danger of being able to act immorally is already ethically very problematic according to the Church. Just think of drinking a little alcohol, which can lead to more and more. Getting drunk is a devastating act against one’s own rational nature, according to some Thomists. One should please explain this to the Catholic Bavarians, who like to have one too many (I don't think they'll ever confess in church their heavy drinking). Not only alcohol but all intoxicating substances outside a medical indication also have no place in natural law.

On the other side. What speaks against an intoxication, which causes a beneficial mental restart. If you had so much stress because you always had to be so logical, conscientious, and sensible at work or with your family, what speaks against a short-term irrational exhilaration which at the end has a very stabilizing effect on one's whole being (of course only when one does not exaggerate it and does it only from time to time)?

The problem of chewing gum

Sex with a natural rubber condom and chewing natural rubber are both cases of a frustration of a biological function if one plays the game of naturalistic natural law.

Now I just want to have fun with two functions. That is, I don't want to eat anything and I don't want a child either. So I'd like to avoid these. So there is something rubbery in my two actions. This rubber stuff leads to the fact that the goals of the functions cannot be achieved. So neither digestion (better put, swallowing down) nor procreation (better put, ejaculating into the vagina).

In both cases, I have to deliberately neglect the functions for the sake of pure fun. I activate a natural faculty without respecting its function. The realization of the goal of both functions is undermined in a deliberate way. The condom puts the activated reproductive ability somehow into a certain non-recurring and one-time run into emptiness. The chewing gum does the same to some extent, but as a continuous idle run. If you look at it generally, which one should do philosophically, then there is no difference between the two cases. Chewing up and swallowing down does not occur, but this should normally be the case when the nutritional function is activated. And ejaculation in the vagina does not occur either, which should also happen when the reproductive function is in progress.

The chewing gum in the mouth is analogous to the sperm in the condom. Or rather, keeping chewing gum in your mouth instead of spitting it out as something indigestible and continuing to chew it is analogous to ejaculating into the condom. I should not have put chewing gum in my mouth from the outset to chew it pleasurably, because it is not digestible and is therefore contrary to nature. Spitting out is, in this case, the telos. However, reaching this telos is destroyed and annihilated by continuing to chew. In addition, the production of saliva is stimulated completely in vain (at least as far as the main function of eating and digesting is concerned), and the stomach is hoodwinked because it anticipates the bites of food in a growling manner. If one was already a little hungry, a negative consequence appears, which expresses itself in slight to moderate pain of hunger. Thus, the act of chewing gum seriously violates natural law.

So in both cases, I want to use a natural faculty to achieve a certain end, but which end is not the natural end. In order to achieve that particular end, I must avoid the realization of the real natural end.

So chewing a nutrient-free gum perverts the eating and digestive faculties. And the digestive function also includes crushing mastication and moistening.

If one does not agree with the analogy presented, then the following applies: The swallowing of chewed and saliva-moistened food into the stomach is comparable to ejaculating in the vagina. Hence: The mere chewing and moistening of food and the immediate spitting out of it is like ejaculating into a condom. Both cases should be given equal moral weight (I consider this to be irrefutable). The interesting thing is that for our moral sense it's enough to spit out the food if it doesn't taste good at all. A completely trivial reason justifies the total perversion. The same should apply to the sex act.

To sum up:

A: Activation of the food intake system.
Avoidance of the system's purpose by chewing a (chewing) gum.

B: Activation of the ejaculation system.
Avoidance of the system's purpose by using a condom.

That chewing gum would also be a serious moral violation is really very ridiculous. And because this is so, sex with a condom is just as morally marginal.

The problem of the fun of a milk-producing woman

Sperm production is involuntarily and ongoing, as well as milk production of a breastfeeding mother. A manual intervention allows both the milk and the sperm to be released. Some men can ejaculate in a few seconds via masturbation, even without the penis being properly erected, all just for fun. And women can squeeze out their milk a bit faster, also just for fun. In both cases, the hands were used to consciously let a liquid (which in both cases is important for the preservation of mankind) escape from the body, a liquid that has a special, functional organic end vessel in another person. If some sperm comes out of the vagina during sexual intercourse and is lost, this is not bad according to the Thomist, nor is it bad if some milk is lost during breastfeeding, for example when the baby is spitting out. But these cases are something completely different because we are talking about cases where the human fluid receptacles are absent. The mere squirting of milk for fun would have to be regarded as a very serious sin. But that would be bizarre.

The problem of moral incentives

What are the actual motivating forces that spur you on to natural law action? This question is not unimportant since nothing good can be done without such impulses. Feser mentions disgust at least once. That this is rather a questionable moral force is proven by the history of discrimination against mixed marriages, in which the feeling of disgust was decisive. Today’s generation also feels a lot of disgust for things that were considered "appealing" at one time. For Schopenhauer himself, there is only one moral driving incentive. As in Buddhism and actually also in Christianity, it is pity or empathy. Here a description of Schopenhauer, which shows compassion is the only true driving force in morality:
Nothing outrages our moral feeling in its deepest ground so much as cruelty. We can forgive every other crime, but cruelty alone we cannot. The ground for this is that cruelty is the direct opposite of compassion. If we are informed of a very cruel deed, as is, e.g., the one that the newspapers are reporting just now about a mother who murdered her five-year-old boy by pouring boiling oil down his throat and her younger child by burying it alive; or the one that is just reported from Algiers, that after a chance dispute and fight between a Spaniard and an Algerian, the latter, being the stronger, tore the other man’s whole lower jaw bone clean off and carried it away as a trophy, abandoning him still alive – then we are seized with horror and cry out: ‘How is it possible to do such a thing?’ – What is the sense of this question? Is it perhaps: How is it possible to fear so little the punishments of the future life? – Hardly. – Or: How is it possible to act on a maxim that is so highly unsuited to becoming a universal law for all rational beings? – Certainly not. – Or: How is it possible to be so negligent of one’s own perfection and that of others? – Equally not. – The sense of that question is quite certainly simply this: How is it possible to be so much without compassion? – Thus it is the greatest lack of compassion that impresses upon a deed the most profound moral reprehensibility and hatefulness. Consequently compassion is the real moral incentive. (Arthur Schopenhauer – The Two Fundamental Problems of Ethics)
One might as well add the natural law principle to Schopenhauer’s list of implausible moral meanings. What about an act of compassion, but not in accordance with natural law? Would that be an impossibility?

Peter Geach draws attention to another problem:
He says,
calling a thing a good A does not influence choice unless the one who is choosing happens to want an A; and this influence on action is not the logically primary force of the word ‘good‘. ‘You have ants in your pants‘, which obviously has a primarily descriptive force, is far closer to affecting action than many uses of the term ‘good‘.
In this passage, Geach himself acknowledges that simply to call something "good" does not, in itself, provide motivation to pursue it. Just as calling a can of sardines "good" will not induce agents to eat them unless they already desire to eat them, so calling certain traits "good" and "part of the essence of the human person" will not induce agents to seek to attain those traits unless they already have a desire to do so. (Shalina Stilley: Natural Law Theory and the "Is"--"Ought" Problem - A Critique of Four Solutions
https://epublications.marquette.edu/cgi/viewcontent.cgi?article=1059&context=dissertations_mu)
And since the Thomists identify "good" and "ought" extremely brazenly with each other (just because a film is good, for example, doesn't mean that everyone ought to see it), one must consider the following:
That ought has any sense and meaning at all only in relation to threatened punishment or promised reward. Thus, long before Kant was thought of, Locke already says:
‘For since it would be utterly in vain, to suppose a rule set to the free actions of man, without annexing to it some enforcement of good and evil to determine his will; we must, where-ever we suppose a law, suppose also some reward or punishment annexed to that law.’ (On Understanding)
So the ought is necessarily conditioned by punishment or reward, and therefore, to speak Kant’s language, essentially and unavoidably hypothetical and never, as he asserts, categorical. (Arthur Schopenhauer – The Two Fundamental Problems of Ethics)
If a punishing or rewarding God is to stand behind what ought to be, then we have no noble ethics. We'd be like rats on a sinking ship. All that matters is that every rat only saves its own butt. Here is a fitting Schopenhauer quote:
[T]heism may provide support for morals, but support of the crudest kind, indeed, one through which the true and pure morality of conduct basically is abolished, since because of it every unselfish action is immediately transformed into a selfish one by means of a very long-sighted, but secure bill of exchange that we receive as payment for it. For God, who in the beginning was the creator, appears in the end as revenger and rewarder. Deference to such a God admittedly can produce virtuous actions; however, because fear of punishment or hope for reward are their motive, these actions will not be purely moral; on the contrary, the inner essence of such virtue will amount to prudent and carefully calculating egoism. Ultimately the firmness of the belief in indemonstrable things is what alone counts; if this is present, then we shall certainly not hesitate to accept a short period of suffering for an eternity of joy, and the actual guiding principle of morals will be: ‘to be able to wait’. However, everyone who seeks a reward for his deeds, be it in this world or a future one, is an egoist; if he misses the hoped-for reward, it does not matter whether it happens through chance, which rules this world, or through the emptiness of the delusion that built the future world for him. For this reason, Kant’s moral theology also undermines morality. (Schopenhauer - Parerga and paralipomena : short philosophical essays.)
A more Nietzschean view of things

If I understand me as a whole, as a teleological whole, then this whole expresses itself in my conscious will. All my parts, which are imperfect in themselves, work only in the service of this perfect will. In order to find my true will, I must free myself from all previous conventions, ideas, and standards. Why should I compare myself to others? After all, it’s all about me. This new natural right would require me to find my true self and disregard everything else. If someone with a theologian’s blood points out to me his nature, namely a priestly nature, as valid for everyone, I will only wrinkle my nose.

The ways of the Thomists are unfathomable

The Thomists seem to know a lot about human organs and faculties. At least they show a lot of self-confidence about it. They should, therefore, be able to create a complete chart listing all organs and faculties with their main functions. This would be very helpful for anyone who wants to understand their theory better. This chart should also list all cases of misuse in accordance with Feser’s "contrary to" criterion and all neutral uses in accordance with his "other than" criterion.

If the Thomist says that sexuality has only one main purpose, then one has to ask him (because the person who makes an assertion has the burden of proof) how he knows that and what he understands by sexuality. On the one hand, the Thomist can say: I know it intuitively. This could be an expression to hide only one’s own prejudices. In addition, another person could say that he or she intuitively sees it differently. Thomists want to sell their views secretly and manipulatively as default views. On the other hand, Feser could say that it is an empirical fact. But what empirical facts does Feser assume? Crude facts about pure external anatomy and physiology? Because he knows about a possible result of a sexual act, namely pregnancy? But in this way, one would never have thought that bonobos have other natural uses of their sexual faculties. Behavioral patterns and internal dispositions must be included in the assessment of biological functionalities. And perhaps one would have to be able to completely decipher the genetic material in order to arrive at correct judgments.

On the question of what sex is, the Thomist would insist only on his conservative ideas, while another would oppose them with his liberal ideas. Human sexuality is very complex: Flirting, falling in love, eroticism, touching of erogenous zones on the body, the mere hearing of the voice of the beloved and so on, and so forth; so many abilities and organs (could) belong to sexuality. To say that a partial activation of this complex must go along with the attempt to conceive a child is more than ludicrous.

One will not be able to change the mind of a Thomist, because the whole functional matter with human bodies is very complex so that it only amounts to one thing: He says so, but I say no, it’s different, it’s like this. The Thomist again says no, and so on.

You can say from every part of the body down to the parts of a cell that it has an end and a means, that it is an end and a means. And each part I can subordinate to a higher organic order. The chewing function can be subordinated to the digestive function and the digestive function to the self-preservation function, which, in turn, can be subordinated to the holiness function. If I pretend to chew something by grinding my teeth together, I would pervert the chewing function, but maybe not the digestive function. Maybe I would perform the muscle function in my mouth and jaw correctly and not really do anything wrong. It would be trivial to say that the ability to chew would be directed to chew, the ability to procreate directed at procreating, the ability to see directed to see. I could say the ejaculation ability is geared towards ejaculation and that’s about it. Or the ability of the penis to erect is directed at erection. Here you can build countless hierarchies up to the very top, to the human being as a whole. I can scratch myself and destroy countless teleological skin cells, all no problem because my well-being has justified it. The hierarchical system allows you to pervert the subordinate element in favor of the higher one. So I could easily say that the generative faculty is subordinate to the sexual faculty.

Most reasons for specific aspects wander only in a circle. How do I know what the right inclinations are? Well, look at anatomy and physiology. How do I know that anatomy and physiology give all the information about human nature? Well, just look inwards, see what you feel, what you have an inclination for. And so on and so forth.

The Thomist talks a lot about straw men and so on when his views are attacked, but he must realize that (with many concessions on our part in other aspects) if his conceptual distinctions are not tenable, the straw men will quickly become steel men. Only small changes in the interpretation of human nature are necessary to completely turn the Thomist's theory upside down.

When assessing biological organs and faculties, one should consider them as generally as possible in order to be able to compare them. If someone invokes individual specifics to make an analogy impossible, a never-ending discussion begins in which only absolute experts in anatomy, physiology, and biology are allowed to participate. At the end of the day these faculties are not identical, but when we philosophize we should only consider one common element so that all faculties are in a clear analogy.

I suspect that the defenders of the pervert faculty argument secretly consider that the sexual and generative faculties of humans are absolutely exclusive and incomparable to anything else in humans. Thus, each objection is pushed aside by the defenders. That’s my estimation. But then one could accuse them of dogmatism. The Thomists must disregard any physiological peculiarity of the sexual organs, otherwise, they are guilty of dishonesty. Sometimes I also have the feeling that in some controversial cases the natural law advocates tacitly resort to an ethical particularism (which they do not actually desire, they certainly want to have general principles). Moreover, the Thomist's conceptual distinctions seem very adapted to sexuality as he imagines it, and very ad hoc constructed concerning the sexual act. This impression cannot be avoided.

If the Thomists are like Aristotle in the sense that, like him, they are very bad observers of nature and actually wish to deduce precepts very quickly in their quiet little room, then good night with natural law.

For Aristotle has made many observational errors:
Besides, the mistakes are too numerous and too gross to be condoned. According to Aristotle males have more teeth than females, not only among mankind but among sheep, goats and swine. According to Aristotle there is a species of ox which has a bone in its heart. According to Aristotle the blood in the lower parts of the body is blacker and thicker than in the upper; the blood of a woman is thicker and blacker than that of a man; therefore a man is nobler than a woman, and the upper parts of the body nobler than the lower. Any butcher or soldier might have taught him better than that. In his credulity, however, he serves up still more fabulous tales. The hen partridge becomes impregnated if the wind blows from the direction of the male bird; at certain times the same effect is produced simply by the cry of the male bird flying over her. The bite of a mad dog produces rabies in every animal with the sole exception of man. His tendency to draw logical conclusions rather than to observe nature is incorrigible. (Fritz Mauthner - Aristotle https://archive.org/details/aristotle00maut/page/10)
Aristotle was not a true scientist:
In brief the truth is this. Aristotle was not an observer of nature, because he had eyes for books only; for that which, in the petty language of bookworms is called, feebly enough, the Book of Nature, he had no eyes at all. He was the first Bibliophile whose name occurs in the tradition of the history of learning. Plato called him the "Reader", making fun of his booklearning in a manner congenial to Plato's poetical spirit. (Fritz Mauthner - Aristotle https://archive.org/details/aristotle00maut/page/10)
Why do Thomists look counterfactually into human life when they want to determine the nature of man? That makes no sense. Therefore, many of their assumptions are contrary to those of evolutionary psychologists, who really strive to capture human nature scientifically. Thomists, when making ethical theses, seem to make moralistic fallacies that are similar to naturalistic fallacies. With the moralistic fallacy, however, the opposite is the case: from what ought to be we draw conclusions about what is. X should be. That is why X is. X should not be. Therefore, X is not. This occurs when, contrary to empirical experience, human nature is inferred from beliefs and worldviews. A concrete example: Adultery and promiscuity are wrong. Consequently, we have no biological predisposition for several sexual partners. While adultery and promiscuity may be considered morally wrong, this has no influence on the biological aspects of humans, which could include promiscuity. The Thomistic natural lawyer simply does not want to see the nature of man as the evolutionary psychologist sees it.

Natural law makes biblical moral commandments superfluous. For whom then are the biblical commandments, if natural law is to be so clear and unambiguous after all? For the less intelligent? Do Thomists actually advocate a philosophical education for all people? Do they have a democratic educational ideal? Strangely enough, according to Aquinas, natural law is intended only for spiritual, intellectual people. The common people must adhere to revelations. But how can an ethical theory that claims general validity for everyone be just one for a few specialists?

The natural lawyer would have to show how to make a list of moral commandments in a believable hierarchical order (for example murder is the worst offense and, therefore, has the highest priority, then follows theft, then comes sodomy, etc.). No one would say that all possible transgressions of natural law that man can commit would be morally equivalent. That would have absurd consequences. A further question arises when considering which misconduct would be a matter for the state, i.e. a criminal offense. Should adultery again become a serious illegal offense? Or even homosexual anal intercourse? Should the attempted suicide be punished again? Masturbation would then perhaps have to be punished with the death penalty? All this would be crazy. From the moment on when the Church along with the Thomists wants to establish its natural law as state law, one would be morally called upon to revolt. Can natural law tell us which form of society is best? Perhaps socialism/communism fits best with natural law (think of Plato’s state, in which man cannot be free exactly because of his nature)?

A man should not divorce, especially if he has just fathered a child. Without him, his child would quickly become poor and miserable. So says Thomistic ethics. But if the man suddenly wants to lead a holy life, he can leave his wife and child immediately without any problems. A holy life justifies the impoverishment of the child, about which the natural lawyers were concerned to the highest degree shortly before. This includes a very bizarre moral rationale. The end justifies the means here. If there is a highest purpose for human nature, as Thomism claims, namely holiness, then this purpose should not be disregarded by anyone. The following should apply to everyone:
Go, sell everything you have and give to the poor, and you will have treasure in heaven. Then come, follow me. (Mark 10:21) 
If my innermost being strives for holiness, and if holiness excludes a family life, then I pervert my inner nature with starting a family. The highest purpose cannot simply be an option. Because it corresponds in the last consequence to our innermost being, at least according to Thomism. Then why should the Christian marry at all?

Jesus has already clearly demanded to leave one's family:
This new thing that was coming, then, required a complete commitment to love God and one's neighbor as oneself, even to the point of abandoning all else—including one's own family and home—in order to do so. (Bart D. Ehrman - Jesus apocalyptic prophet of the new millennium)
And:
The Jesus of history, contrary to a modern "common sense" (at least in large chunks of American Christianity), was not a proponent of "family values." He urged his followers to abandon their homes and forsake families for the sake of the Kingdom that was soon to arrive. (Bart D. Ehrman - Jesus apocalyptic prophet of the new millennium)
Strangely enough, the church gives man a choice between two options:
It is pivotally important to historical Thomism to note that the choice between having a sexual life and having a life of contemplation were thought of as exclusive alternatives, in a way Aristotle certainly never thought of them. (Nicholas Bamforth, David A. J. Richards – PATRIARCHAL RELIGION, SEXUALITY, AND GENDER)
But according to Catholic dogma, however, virginity is higher in value than fertile married life:
We have already shown that this Augustinian teaching is found expressed by Canisius and by the Council of Trent as the invariable belief of the Church. But that it has been retained till the present day as a dogma may be sufficiently established by the June 1831 number of the periodical Der Katholik. On p. 263 it says: ‘In Catholicism the observance of a pervetual chastity, for God’s sake, appears in itself as the highest merit of man. The view that the observance of perpetual chastity as an end in itself sanctifies and exalts man, is, as every instructed Catholic is convinced, deep-rooted in Christianity according to its spirit and its express precept. The Council of Trent has removed all possible doubt about this.’ It must certainly be admitted by every unbiassed person not only that the teaching expressed by Der Katholik is really Catholic, but also that the arguments adduced may be absolutely irrefutable for a Catholic’s faculty of reason, as they are drawn directly from the fundamental ecclesiastical view of the Church on life and its destiny.” Further, it is said on p. 270 of the same work: “Although Paul describes the prohibition to marry as a false teaching, and the even more Judaistic author of the Epistle to the Hebrews enjoins that ‘Marriage shall be honourable in all, and the marriage bed undefiled’ (Hebr. xiii, 4), yet the main tendency of these two sacred writers must not on this account be misunderstood. To both virginity was perfection, marriage only a makeshift for the weaker, and only as such was it to be held inviolate. The highest endeavour, on the other hand, was directed to complete, material casting off of self. The self should turn away and refrain from everything that contributes only to its pleasure and to this only temporarily.” Finally on p. 288: “We agree with the Abbé Zaccaria, who asserts that celibacy (not the law of celibacy) is derived above all from the teaching of Christ and of the Apostle Paul.” (Arthur Schopenhauer: The World as Will and Representation: CHAPTER XLVIII: On the Doctrine of the Denial of the Will-to-Live)
Catholicism and thus the majority of Thomism seems to me to be in a schizophrenic and illogical situation between natalism commanded by God (Old Testament) and holy antinatalism (New Testament). But whoever decides for natalism runs the risk of becoming an everyday Christian:
If the Christian dogmas of a revengeful God, universal sinfulness, election by divine grace, and the danger of eternal damnation were true, it would be a sign of weak-mindedness and lack of character not to become a priest, apostle or hermit and, in fear and trembling, to work solely on one's own salvation; it would be senseless to lose sight of ones eternal advantage for the sake of temporal comfort. If we may assume that these things are at any rate believed true, then the everyday Christian cuts a miserable figure; he is a man who really cannot count to three, and who precisely on account of his spiritual imbecility does not deserve to be punished so harshly as Christianity promises to punish him. (Friedrich Wilhelm Nietzsche - Human All-Too-Human, A Book For Free Spirits)
In many places Thomas Aquinas argues like a consequentialist:
The pleasures in sexual love having, for Thomas, no value whatsoever, he ascribed to sexuality a purely instrumental good, consistent with God’s larger purposes for humankind: namely, procreation, but procreation linked to the kinds of care and nurture of the young of the human species required for their proper development. Marriage was the only acceptable form of such procreative unions for completely consequentialist reasons: Only an indivisible marital unionreasonably secured the kind of long-standing relationship between a man and a woman consistent with appropriate kinds of care. Single-parent motherhood would not be justified because, as Thomas read the facts of gender difference: “a woman alone is not adequate to this task; rather this demands the work of a husband, in whom reason is more developed for giving instruction and strength is more available for giving punishment.” Monogamy between a man and a woman was also required for comparable consequentialist reasons. Men would have no incentive to commit themselves to a long-standing relationship to a woman “if there were several males for one female.” And while several females with one man would satisfy this requirement, it would frustrate the desires of women, as of men, shared by animals and humans, to have an unimpeded liberty of access to a sexual partner. Finally, monogamous relationships were preferred for a further consequentialist reason: “friendship consists in an equality,” and polygamy led, as Thomas argued experience demonstrated, to a situation where “the friendship of wife for husband would not be free, but somewhat servile.” For Thomas, such consequentialist arguments are the most reasonable way, assuming Aristotelian science and ethics, to construe what the teleological aims of a just God are for us. It would violate the whole tenor and spirit of Thomas’s rigorously scientific and philosophical argument to reverse the intellectual order of the argument, making a fixed sectarian conception of teleology the premise of the argument independent of good arguments of science and philosophy. (Nicholas Bamforth, David A. J. Richards – PATRIARCHAL RELIGION, SEXUALITY, AND GENDER)
And:
It is Thomas’s methodological concern with reason – in particular, the concern to justify his first part arguments by reference to what was understood to be the most reasonable science and philosophy available in his day (for example, drawing a distinction between knowledge acquired through reason and through revelation) – that is the obviously appealing feature of his approach. His views concerning, for example, the legitimate (and limited) role of sexual activity were thus directly tied to his belief in the importance of a contemplative life freed from bodily distraction, and must be seen in the light of that belief. An appealing contemporary interpretation of Thomas must surely, therefore, be one which prioritises his methodology over many of his substantive conclusions: Its arguments must be based upon the best available science and philosophy of the day (abandoning where necessary Thomas’s substantive conclusions if they do not match up to such a standard), and its conclusions justified accordingly. (Nicholas Bamforth, David A. J. Richards – PATRIARCHAL RELIGION, SEXUALITY, AND GENDER)
Then:
Thomas thus believed that his identification of goods for persons arose from a naturalistic understanding of the rational desires and competences of persons, an understanding that arose from and was consistent with the best science available at the time he was writing. His perfectionism assigned value and weight to these goods, in order to promote the fullest overall realization of them (consistent with the perfectionist principle that maximizes the display and flourishing of human excellences). Within the framework of his naturalistic understanding of science and of ethics, Thomas was clearly reasoning in a consequentialist fashion; indeed, he seems to have been just as concerned with overall consequences as later thinkers such as Bentham. Nonetheless, it is also abundantly clear that some of Thomas’s substantive positions – in particular, his views concerning gender, sexuality, and religious dissent – are simply anachronistic when viewed from a modern naturalistic standpoint using, as Thomas’s methodology demands, the best contemporary forms of knowledge. (Nicholas Bamforth, David A. J. Richards – PATRIARCHAL RELIGION, SEXUALITY, AND GENDER)
Finally a curiosity by Aquinas:
Against brother-sister incest there is a very curious argument: that if the love of husband and wife were combined with that of brother and sister, mutual attraction would be so strong as to cause unduly frequent intercourse. (Russell, Bertrand - History of Western Philosophy)
What exactly is sex?

The Thomists say when sex can become morally bad without being able to say beforehand with absolute precision(!) what sex is. That's presumptuous. They cannot dogmatically assume a Platonic idea of sexuality without being able to fully define it.

What if I fall intensely in love and resist this (romantic, but also sexual) urge for superstitious or other reasons? Is that compatible with natural law? Is falling in love, which I have consciously allowed, already the point of no return in the teleology of sexuality, as Catholicism understands it, and thus also of marriage? The determination of such a point seems to be an impossible undertaking. And yet the natural lawyer needs this point, from which there is no turning back during the bodily approach between man and woman, where there is no possibility to say stop, and consequently, only the duty exists to direct the semen as soon as possible into the vagina, otherwise, it would not be a problem if Tantra grandmasters have an orgasm during sex without ejaculating (dry orgasm). For
orgasm and ejaculation are two totally different processes. This means that as a man you can have an orgasm but not ejaculate. (https://www.health24.com/Sex/Problems/Sexual-superhero-orgasm-and-ejaculation-20120721) (www.shedoesthecity.com/how_to_orgasm_without_ejaculating)
According to Feser, sex begins (very imprecisely) with sexual arousal (an inner state) and ends with ejaculation in the vagina (a purely external event). Unfortunately, Feser’s definition is of no use at all. To say that sex begins with sexual arousal would be as ridiculous as to say that sex consists only of the very short moment of ejaculation (in the vagina). And yet Feser must choose the latter. If he does not decide in favor of the latter, then he will have great argumentative difficulties, as we shall see.

Feser, for example, makes questionable concessions in his version of natural law. According to Feser in his(!) Last Superstition, oral sex can be part of the sexual prelude without any problems, as long as the ejaculation happens at the end in the vagina. Feser does not mention the hated anal sex, but for theoretical reasons, it must not be excluded, even if it is rather unusual (even in the protected mode) as foreplay. But when I start foreplay, do I have to finish sex with ejaculation in the vagina? Actually, yes, if foreplay is an essential part of sex. Or doesn't it belong to it?

What if I urgently have to urinate during sexual intercourse, then surely I should also be free to pursue this urge and go to the toilet. But when I come back and the sexual mood is down, do I still have to finish the sexual act? If during sexual intercourse it occurs to me that I have an important appointment and have to go immediately, can I stop having sex? A fire suddenly breaks out, the corresponding alarm goes off, the baby screams and someone knocks on the door and quickly calls you in and with all this I am on the verge of climax or even in the middle of it, then I can certainly stop the act in which I have just been, even if a possible ejaculation should completely miss its target. According to which criteria should one be able to say when an abruption was legitimate or not?

What about lesbian sexual love? It should not be negatively affected by natural law. Because it would be nothing more than an exchange of tenderness that could hardly be convincingly morally forbidden.

Do women possibly have a special status under natural law because their clitoris does not directly contribute to pregnancy? Is there possibly no equality in natural law if the nature of women is slightly different from that of men?

One can also look at the phenomenon of tongue kissing. The tongue kiss may have a slightly different purpose for the man than for the woman. For the man, it is a means to make the woman submissive to the sexual act. For the woman it is, so to speak, a kind of tasting of whether the man is Mr. Right for a long-term relationship. So under Feser’s conditions, the executed and correctly completed sexual act would not necessarily be obligatory for a woman as it is for a man. If the "chemistry isn’t right", she could quickly veto it and prevent any form of physical contact.

Speaking of kissing, we can wonder what the mouth is for:
For instance is the purpose of a mouth for eating or for kissing or for both? Who is to decide? If kissing is part of the function of mouths, then kissing would become a good rather than, arguably, an evil. The need to make assumptions which may be challenged is, therefore, implicit in Aquinas’ whole approach and weakens its effectiveness. (Vardy, Peter. The Puzzle of Ethics)
What about intersexuals who are neither man nor woman? Intersexuals should be able to sexually satisfy themselves. There is nothing wrong with enjoyment. If they have that right, then everyone should have it.

What about men who have two penises (The Penis of an Incorporated Vanished Twin Brother)? This phenomenon actually exists. In some cases, both penises can be sexually stimulated, but only one is suitable for reproduction. 

What would the natural lawyer say to the man who, through pure erotic fantasy, can bring himself to ejaculate without somehow physically touching himself? Is anything perverted here? You can have fantasies, and ejaculation is more of a physical side effect like nocturnal emissions.

What about the case when I, as a man, am in deep sleep and another person starts to stimulate my genitals sexually with his hand without me waking up. This only happens when I ejaculate. Has this person perverted my generative faculty?

What about karezza or coitus reservatus, a sexual practice in which the man consciously and willingly refrains from ejaculation? Although the conventional orgasm is omitted as the climax of sexual intercourse, the decoupling from ejaculation results in a sustained "orgiastic" state. 

What about micro-penises in sexual action?

What Grisez notes here regarding artificially induced vomiting (which would be a serious offense under natural law) can be applied to many other cases:
[A]s soon as there is any good reason to induce vomiting, no objection is made to doing so. For example, even a small danger that one has consumed poison or a moderate discomfort which may be relieved by vomiting are sufficient justifications for inducing it. (Germain Grisez: Contraception and the New Natural Law – Chapter 2: INADEQUATE ARGUMENTS https://catholicebooks.wordpress.com/2014/07/25/online-text-contraception-and-the-natural-law-by-germain-grisez/)
And this shows, according to Grisez, that it is not the perversion itself that we morally reject. But the perverted faculty argument presupposes that a perversion of a faculty is morally (intrinsically) wrong and must be felt as immoral, otherwise the name of the argument no longer makes sense. A couple who are currently sleeping together and are suddenly disturbed by their screaming baby in the next room will of course immediately go to their baby and end their sexual intercourse. Maybe the man was only able to let his semen come out on the bedsheet. The couple won’t think: "Oh boy, we just perverted our sexual faculty, hopefully, the little one has something serious and doesn’t just whine." They will not be burdened in the least by their moral conscience and nobody would blame them for anything. It seems so easy to undermine the moral heaviness which, according to the Thomists, lies in the order of the sexual act.

Thomists should definitely not talk about perverted sexuality, but only about the perversion of men's fertile ejaculation system. 

There could be situations which, as we have seen, occur just before the already induced ejaculation, justifying the penis being removed from the vagina so that ejaculation takes place completely outside the vagina. The Thomist should now state the official criteria for that justification of the ejaculation disorder. That would be very interesting. 


On the function of the hand

Let's start with a Schopenhauer quote. We know that Schopenhauer is not a natural lawyer:
Therefore the will-to-know, objectively perceived, is the brain, just as the will- to-walk, objectively perceived, is the foot; the will-to-grasp, the hand; the will-to-digest, the stomach; the will-to-procreate, the genitals, and so on. (Arthur Schopenhauer - CHAPTER XX, The World as Will and Representation Volume II)
Thus, hands would be very limited in their function under natural law. Perhaps their function really consists only of grasping. The Thomist cannot rule this out with absolute certainty. Or could it not be said that the hands have several, very special, main functions depending on the situation, such as grasping, striking, stroking, scratching and pricking/poking with the fingers, signaling, and, culturally new, welcoming? Anyone who says that the hand has no special function, that it is rather of universal use, is guilty of an excuse and embarrassment. The fact that the hand can be used universally is due to its random anatomy. For the same reason, the eyes cannot be used universally, but in my imagination, I can conceive of eyes with much wider use.

Homosexuality, exercised

The teleology of a partnership, a loving partnership in which one should meet at eye level, is perhaps more realized with homosexual couples than with heterosexuals. Just think of Plato, for whom there can only be true love between men. One could say that homosexual couples miss the natural law in sexual respect, but fulfill it in respect of the partnership. With heterosexual couples, it could be exactly the other way round (if you doubt it, you only need to see recordings of old talk shows). There should be no reason why the partnership aspect of an official marriage or normal love relationship should not have its own meaningful teleology. Because one must not forget that most of the time of living together is spent without sex.

Although practiced homosexuality is usually rejected by natural law, it cannot reject in theory any culture of homoeroticism (homoerotic tenderness) and thus homoerotic communal living. The homoerotic act itself does not pervert anything. Plato and Aristotle even endorsed same-sex sensual caresses. So to approve of homoeroticism in a certain sense because there are no real arguments against it, but to be morally indignant when it goes a little too far is completely ridiculous.

By the way, would homosexuals sin much less in the eyes of Catholics if they adopted children after their sexual acts? After all, they would at least be closer to the idea of reproduction.

And if, as a man, I act like a sissy, do I also violate natural law because I distance myself from the masculine form?

Schopenhauer can report on homosexual pederasty (old man and teenage boy) in ancient Greece:
The philosophers also speak much more of this [homoerotic] love than of the love of women; in particular, Plato seems to know of hardly any other, and likewise the Stoics, who mention it as worthy of the sage. (Stobaeus, Eclog. eth., bk. II, c. 7.) In the Symposium, Plato even mentions to the credit of Socrates, as an unexampled act of heroism, that he scorned Alcibiades who offered himself to him for the purpose. In Xenophon’s Memorabilia, Socrates speaks of pederasty as a thing blameless and even praiseworthy. (Stobaeus, Florilegium, Vol. I, p. 57.) Likewise in the Memorabilia (Bk. I, cap. 3, § 8), where Socrates warns of the dangers of love, he speaks so exclusively of love of boys that one would imagine there were no women at all. Even Aristotle (Politics, ii, 9) speaks of pederasty as of a usual thing, without censuring it. He mentions that it was held in public esteem by the Celts, that the Cretans and their laws countenanced it as a means against overpopulation, and he recounts (c. 10) the male love-affair of Philolaus the legislator, and so on. (Arthur Schopenhauer - The World as Will and Representation Volume II)
In the following quotations, Otto Weininger in SEX & CHARACTER (http://www.fatuma.net/text/sexchar.pdf) reports that no real forms of the male and the female exist. There are only intermediate stages that can be determined with the help of the purely artificial concepts of the absolutely feminine and the absolutely masculine. However, these terms are only auxiliary terms from which no code of conduct can be derived. Homosexuality is a normal natural phenomenon for Weininger. He said this already around the year 1900. This is the time of Freud, which began to have a closer look at human sexuality. Before that time, this science was still in its infancy:
In the widest treatment of most living things, a blunt separation of them into males or females no longer suffices for the known facts. The limitations of these conceptions have been felt more or less by many writers. The first purpose of this work is to make this point clear. 
Amongst human beings the state of the case is as follows: There exist all sorts of intermediate conditions between male and female – sexual transitional forms. 
Living beings cannot be described bluntly as of one sex or the other. The real world from the point of view of sex may be regarded as swaying between two points, no actual individual being at either point, but somewhere between the two. The task of science is to define the position of any individual between these two points. The absolute conditions at the two extremes are not metaphysical abstractions above or outside the world of experience, but their construction is necessary as a philosophical and practical mode of describing the actual world. 
Let those who regard sexual inversion as pathological, as a hideous anomaly of mental development (the view accepted by the populace), or believe it to be an acquired vice, the result of an execrable seduction, remember that there exist all transitional stages reaching from the most masculine male to the most effeminate male and so on to the sexual invert, the false and true hermaphrodite; and then, on the other side, successively through the sapphist to the virago and so on until the most feminine virgin is reached. 
What is new in my view is that according to it, homosexuality cannot be regarded as an atavism or as due to arrested embryonic development, or incomplete differentiation of sex; it cannot be regarded as an anomaly of rare occurrence interpolating itself in customary complete separation of the sexes. Homo-sexuality is merely the sexual condition of these intermediate sexual forms that stretch from one ideally sexual condition to the other sexual condition. In my view all actual organisms have both homosexuality and heterosexuality. 
That the rudiment of homosexuality, in however weak a form, exists in every human being, corresponding to the greater or smaller development of the characters of the opposite sex, is proved conclusively from the fact that in the adolescent stage, while there is still a considerable amount of undifferentiated sexuality, and before the internal secretions have exerted their stimulating force, passionate attachments with a sensual side are the rule amongst boys as well as amongst girls. 
There is no friendship between men that has not an element of sexuality in it, however little accentuated it may be in the nature of the friendship, and however painful the idea of the sexual element would be. But it is enough to remember that there can be no friendship unless there has been some attraction to draw the men together. Much of the affection, protection, and nepotism between men is due to the presence of unsuspected sexual compatibility.
Maybe Aristotle was even gay:
Aristotle was Plato’s most genuine disciple; he spoke with a lisp, as we learn from Timotheus the Athenian in his book On Lives; further, his calves were slender (so they say), his eyes small, and he was conspicuous by his attire, his rings, and the cut of his hair. According to Timaeus, he had a son by Herpyllis, his concubine, who was also called Nicomachus. […] He seceded from the Academy while Plato was still alive. Hence the remark attributed to the latter: “Aristotle spurns me, as colts kick out at the mother who bore them.” He also taught his pupils to discourse upon a set theme, besides practising them in oratory. Afterwards, however, he departed to Hermias the eunuch, who was tyrant of Atarneus, and there is one story that he was on very affectionate terms[!] with Hermias [in German, this is translated as "some claim that he was his lover"][.] (DIOGENES LAERTIUS – LIVES OF THE EMINENT PHILOSOPHERS)
And by the way, Aquinas may have been a greedy glutton:
The Dominican monk Thomas Aquinas was so impressively corpulent that a round piece had to be sawn out of his desk so that he could sit and work on it. The carpenter was then able to attach the cut-out piece to the desk of a very lean monk. (from a German anecdote book about philosophers: Geh mir aus der Sonne! Anekdoten über Philosophen und andere Denker. Hrsg. Von Peter Köhler)
Homosexuality is said to be bad because it lacks procreative potential:
Catholic doctrine has not made explicit the implications of its reasoning – namely, that procreative potential is an absolute moral requirement for all sexual intercourse. The natural law argument in its classical form implicitly ruled out any intercourse, even in marriage, which will not lead to conception, whether on account of age, pregnancy, or permanent or temporary infertility. The latter applies to most of the female’s monthly cycle. One may be forgiven for wondering why this procreative requirement has become the basis for saying that all homogenital acts are unnatural and hence objectively wrong and immoral. Given what we now know about the nature and irreversibility of homosexuality, the Catholic position seems selective in emphasizing the procreative function of sexual organs over the ‘given’ (i.e. natural) physical, emotional and psychological facts about sexual attraction. Why the emphasis must remain on reproduction has never been made clear. To be fair, Aquinas did make a distinction between those deprivations of procreative potential which essentially lack such potential and those that only accidentally lack it. In the Summa Contra Gentiles, Thomas stipulates that any genital activity that allows for no possibility of insemination (the depositing of semen in the vagina) is regarded as essentially nonprocreative and hence unnatural. While heterosexual infertility is deemed only ‘accidentally’ nonprocreative, homogenital acts or contraceptive acts are ‘essentially’ non-procreative. This distinction appears somewhat academic, however. For instance, few would claim that insemination continues to have the same ‘essentially’ procreative potential after the female partner is postmenopausal, or between a married couple when one partner is infertile. Yet there is no suggestion from the Vatican that post-menopausal intercourse, or intercourse between a fertile and an infertile spouse is in any sense immoral. The essential-accidental distinction of St. Thomas is unconvincing and casts doubt on whether his conception of natural law provides an adequate basis for the condemnation of homogenital acts as such. (Murray, T. M.. Thinking Straight About Being Gay)
And:
What sense does it make to postulate one type of sexual activity as normal in this way, so that heterosexual intercourse is held to be an act of a reproductive kind even if reproduction is not intended and is known to be impossible? Why is it not equally plausible to say that all acts of seminal ejaculation are reproductive in kind, or to say that no acts of seminal ejaculation are reproductive in kind, and that reproduction is only an accidental consequence that may ensue under certain conditions? There is nothing in nature that dictates that the lines have to be drawn in any of these ways. Moreover, even if heterosexual sex is normal, it does not follow that it is normative. (Andrew Koppelman - The Gay Rights Question)
Finally:
“Biological union between humans,” Finnis writes, “is the inseminatory union of male genital organ with female genital organ; in most circumstances it does not result in generation, but it is the behavior that unites biologically because it is the behavior which, as behavior, is suitable for generation. ”Whether such behavior “is suitable for generation” would seem to depend, however, on whether the organs that are used are suitable for generation. A sterile person’s genitals are no more suitable for generation than an unloaded gun is suitable for shooting. If someone points a gun at me and pulls the trigger, he exhibits the behavior which, as behavior, is suitable for shooting, but it still matters a lot whether the gun is loaded and whether he knows it. Intent matters: the act is a homicidal kind of act even if the actor mistakenly thinks the gun is loaded, when in fact it is not. Material reality matters, too: if, knowing the gun is unloaded, he points it and pulls the trigger, intending homicide, then indeed fantasy has taken leave of reality. But the only aspect of material reality that matters is whether the gun, as it now is, is in fact capable of killing. Contingencies of deception and fright aside, all objects that are not loaded guns are morally equivalent in this context: it is not more wrong, and certainly not closer to homicide, to point a gun known to be unloaded at someone and pull the trigger than it is to point one’s finger and say, “bang!” And if the two acts have the same moral character in this context, why is the same not equally true of, on the one hand, vaginal intercourse between a heterosexual couple who know they cannot reproduce, and on the other, anal or oral sex between any couple? Just as, in the case of the gun, neither act is more homicidal than the other, so in the sexual cases, neither act is more reproductive than the other. One may, finally, insist on the essentialism implied by the ordinary meaning of words. The heart of a dead man, which will never beat again, is still a heart. His stomach is still a digestive organ. Similarly, the penis of a man who is sterile is still a reproductive organ. But the only aspect of reproductiveness that is relevant to the natural lawyers’ argument, namely the reproductive power of the organ, does not inhere in this organ. It is not a reproductive organ in the sense of power or potential, even if it is a reproductive organ in the taxonomic sense. (Andrew Koppelman - The Gay Rights Question)
The following passage is illuminating regarding being gay and at the same time a Thomist:
In the course of my academic work with Thomas Aquinas, I came into contact with many men (and a few women) who were enthusiastic about the thinking of Thomas Aquinas. And not a few of these men were also gay, as I noticed on closer acquaintance. However, only one of the well-known Thomists dared to stand also publicly to it: Mark D. Jordan, perhaps the most talented of the American Thomas researchers. Jordan, in contrast to the many Thomists from the clergy, had the advantage that he could take this step without taking any economic risk. Originally from the conservative milieu and still employed at the Institute for Medieval Philosophy of the Catholic University of Notre Dame in Indiana, at the time of his outing he already had the prospect of a chair at a non-denominational university. Here his predisposition and his plea for a fundamental change of direction of the Catholic Church with regard to homosexuality could not harm him. Today, Jordan holds the highly prestigious and lucrative Richard Reinhold Niebuhr Chair at Harvard Divinity School, where he continues to give courses in Thomistic philosophy. […] But now back to the question whether there is a connection between homosexuality and the preference for Thomism. I believe today that this question can be answered with an unequivocal yes. The way the Aquinate thinks is to the personal benefit of many homosexual Thomists. His entire philosophy and theology are consistently objectiveist and self-forgetful. He never theologizes "from below", from man, but always "from above", from God and revelation. In contrast to other theologians of church history his person and his individuality play no role in his work. A genre like the famous confessions of the ancient church father Augustine, where he reports on the sins of his youth life, is profoundly opposed to Thomas' way of writing. In his books one searches in vain for religious feelings, piety, doubts or sins of the author. That might also be the reason why a few years after his death in 1274 Thomas could be easily elevated to the holy of all theologians, to "Doctor Angelicus" or angelic teachers. The subject, the theologian himself and his private life, remain completely hidden in Thomism. Typical for it is the Thomistic principle: "The thing must speak, not the person". In a system structured by such a premise one can as a homosexual Catholic theologian without problems think at home without having to withdraw as a gay - and vice versa. Thus here the double life of many gay theologians finds its programmatic anchoring, so to speak, endowed with highest consecrations. Then there would be only the unambiguous statements of Thomas about the "sin of Sodom", as he calls homosexuality in typical medieval language. The more one understands oneself as a Thomist and the more one is titled by others, the more one naturally thinks about this contradiction. But also for this there is an explanation compatible with classical Thomism: One must, as with all great religious writings, distinguish in Thomas' thinking between the substantial central motifs and the subordinate. Just as there are load-bearing foundation walls and foundations in a house that cannot simply be demolished without causing the entire building to collapse, there are also fundamental ideas in Thomas's mental building that are fundamental. These mostly refer on the one hand to structural characteristics of thinking, e.g. the classification of nature and supernaturality, of philosophy and theology, state and church, and on the other hand to his endeavour to bring faith into dialogue with current science and profane thinking. It is precisely in this readiness to engage in dialogue that he proves to be revolutionary and original for his time and still topical for our time. On the other hand, there is that which is of secondary importance. It goes without saying that Thomas talks to the science of his time and therefore arrives at conclusions that are based on the errors of the natural sciences of the time and the social regulations. Here the most important thinker of the Catholic Church reveals himself completely as a child of his time, the High Middle Ages, his thinking as historically conditioned and thus changeable. The theories and their legal consequences that came about in this way are, however, only secondary for Thomas, and he would never have come up with the idea of attributing them supertemporal significance. Some things are no longer relevant today. (my own translation from German, Berger, David - Der heilige Schein: Als schwuler Theologe in der katholischen Kirche, The Holy Pretense: As a Gay Theologian in the Catholic Church)
Here is an addition to the Berger quote. Berger writes that Thomism thinks purely objectivistic. This is a great weakness of Thomism, which has missed the philosophical development of subjectivism (which does not exclude objectivity) since Descartes:
Second, revived Thomism, especially in the works of Maritain and Gilson blocked serious consideration of the Copernican turn so prominent in nineteenth and twentieth century German Catholic thought. Because of Thomistic-Aristotelianism’s intransigent objection to the Copernican Revolution in Philosophy, many important contributions of post-Kantian German Catholic philosophy, especially phenomenology, have been marginalized. It has thereby blocked adequate consideration of interiority or the inner domain. (CAPALDI, NICHOLAS: USING NATURAL LAW TO GUIDE PUBLIC MORALITY - The Blind Leading the Deaf. In: THE DEATH OF METAPHYSICS; THE DEATH OF CULTURE - Epistemology, Metaphysics, and Morality. Edited 2005 by Mark J. Cherry)
Internal attitude or outward appearance

The Christian ethics par excellence, the ethics of the Sermon on the Mount, has almost nothing to do with the ethics of natural law. The Sermon on the Mount proclaims a pure ethics of attitude (and also otherwise, Christian ethics presents itself mainly as an ethics of compassion). One only has to look at the Sermon on the Mount, the passages from Matthew chapter 5,1-7,29 (where murder is not considered substantially different from calling a man a fool, or where adultery is not distinguished from a lustful gaze), then one can ask oneself what they have to do with a naturalistic natural law. Namely, nothing. Also the Golden Rule: "So in everything, do to others what you would have them do to you, for this sums up the Law and the Prophets" (Matthew 7:12) shows no connection. In the Golden Rule, reference is made to one’s own (personal) will, which is precisely not the approach of naturalistic (perverted faculty) natural law ethics.

The Thomists often cannot decide what is more important in action, the mere attitude, or the realization of an external form. Sometimes one seems to be more important than the other, in another context vice versa. Often the Thomists do not make this point clear enough. Everyone who reads their texts should pay close attention to this.

What about, by the way, the one who only adheres strictly to natural law out of fear of hell? Kant would say that his compulsive actions were only outwardly moral, but did not result from a respectful appreciation of moral laws, because there was no genuine moral motive.

Natural law should actually be consistently characterized as something purely external, the observance of which is also something purely external, so that the internal, the motive and attitude of a person is irrelevant. In this way, one would have no difficulty in distinguishing a couple who uses calendars for contraception (fine for Catholics) from another one who uses condoms (sinful for Catholics). Because both couples would be completely identical in their ethos. Thus calendar contraception (rhythm-method) and condom use include different external means, but an identical attitude and intent.

On the other hand, there are no external differences between suicidal martyrdom and "normal" suicide, so that one should not approve of any form of suicide. In comparison: In a "normal" suicide and suicidal martyrdom there is a different attitude, but very similar or identical external means of killing oneself.

There is an imbalance here. The suspicion of arbitrariness arises in relation to the ethical weighting of attitude or intent and the sole upholding of an external order of things.

About natural contraception in connection with intention as the only moral criterion in natural law: The Catholic Church allows so-called natural birth control methods, such as the temperature method, calendar method, checking the consistency of the cervical mucus, perhaps also hormone level measurement. Together they are just as effective as using condoms. I don’t understand the ban here and the permission there at all. If you want to become platonic now, you can say that they all participate in the idea of birth control without exception. To call some natural and others unnatural is not quite convincing. Behind the so-called natural means there is a whole artificial research apparatus and while using them one has to resort to artifacts like computers, calendars, pens for documentation, otherwise, they are ineffective. Condoms are made of natural substances, perhaps there are also natural substances that make fertilization unlikely in women. The only difference consists in an insignificant outside or inside of the body. Thus the thesis that artifacts or other natural things (such as condoms) can never be natural, although animals also use natural things as tools or even produce primitive artifacts, is clearly wrong. The Protestants also saw this in the same way and came to the following conclusion:
All forms of birth control, as far as they are free of health-damaging side effects, are thus permitted - pessary, condom, coitus interruptus and so on, whereby one can refer to the Bible, which does not know such prohibitions, on the other hand [Protestantism] takes the logical view that the use of the days when there is no possibility of conception is in principle not ethically higher than the use of mechanical means. 
(Karlheinz Deschner – Das Kreuz mit der Kirche, The cross with the church)

The Church would also not approve of all people beginning to celebrate orgies with natural birth control means only. The telos of a natural faculty is said to be a moral precept when that faculty is activated. It’s a bit strange then to allow the probability of reaching the telos to be extremely reduced before or as soon as one activates the natural ability. It is like when I want to help a seriously injured person lying on the ground, but intentionally make the help very unlikely. Or, like making a promise only when the commitment can hardly be kept because of previously arranged things.

So one can see that Catholics see the moral center of gravity in a certain order of things, and the intention is only secondary in the sense that it has meaning only with regard to that center. For Protestants, the intention and internal attitude seem to be the most important thing, and thus the order of things would only be peripheral. So depending on the intention, the same act (order of things) could be good or bad.

Natural family planning (NFP) as a contraceptive is now permitted among Catholics. But the pill or the condom remain sinful. Yet the intention of a couple using NFP is exactly the same as that of the one using a condom. A God who makes a moral difference in the two couples must already be a poor God, especially since the effectiveness and safety according to the Pearl Index is higher with NFP than with the condom.

Yet Catholics speak of possible abuse of NFP. 
I love how he mentioned that part of what it means to be married is to be open to life. It seems to me that many Catholics are abusing their use of NFP to limit the number of children they have. They might have economic reasons or whathaveyou that they consider "grave", but in reality, they simply don't want to have any more and make excuses for themselves. I find we in America are particularly prone to that, given the level of comfort most of us enjoy. Even if a couple has a "big family", they'll think they've done their part and stop at 5 or so kids. No, NFP was never meant to be used like that. I would love to hear someone present on the topic of NFP on your channel, Matt. (Some conservative catholic in the youtube comment section: https://www.youtube.com/watch?v=niHB0QzFrrw>)
The quote talks about abuse. One could well say perversion instead. What is perverted then? The intention to the many NFP uses perhaps perverts the reason overall. But how is a secularist supposed to understand all this?

About the martyr's suicide in connection with the realization of an external form as the only moral criterion in natural law: Jesus could justify suicide here (John 15:13) under certain circumstances: "Greater love has no one than this: to lay down one’s life for one’s friends." (Contemporary English Version: The greatest way to show love for friends is to die for them.) The absolute deadly sin in natural law, namely suicide, would under certain circumstances even be morally good. That's odd enough as it is. Suicidal martyrdom might be such a case. Outwardly, pure suicide takes place, which in one way or another destroys a natural order. A natural law that only takes into account the external form of things would be completely demolished by consenting to such death or only by allowing it to happen. Nietzsche on suicide in the context of Christianity:
When Christianity came into being, the craving for suicide was immense—and Christianity turned it into a lever of its power. It allowed only two kinds of suicide, dressed them up with the highest dignity and the highest hopes, and forbade all others in a terrifying manner. Only martyrdom and the ascetic’s slow destruction of his body were permitted. (Friedrich Nietzsche – The Gay Science)
"Other than" and "contrary to" nature

I can use my organs or natural faculties in two ways (both deliberately and with anticipation of what is going to happen) that do not fulfill the true natural purpose of those organs or faculties. This means that when I activate a natural faculty I do not have to meet the actual purpose or end of that faculty, which is a bit odd because the natural purpose is ultimately identified with a good and at the same time with a moral imperative. If this were not the case, then perversion would not be a problem anymore.

Both cases are therefore first of all "other than" cases, where the realization of the actual end does not take place. Something must be added to the morally neutral "other than" use so that it can become an evil "contrary to" one. What can this evil something be? It could be an actively realized harmful order of body or mental parts or just an evil intention, regardless of whether there is any harm.

Here are now "other than" examples that might be helpful to determine what a damaging or harmful use means: playing soccer, cycling, chewing gum, walking on your hands, holding a table upright with one leg, or holding metal nails between your teeth. Walking on your hands can cause serious damage in the long run, as can holding a table upright with one leg or holding metal nails between your teeth. The degrees of damage to the organs, faculties, the human body, mind, and the whole human being always depends on the duration of the action deviating from the natural end. The same applies to playing football and chewing gum. If they take place long and very often, they become harmful. The same goes for sex. A very controlled regular masturbation is not harmful (what's more, I'm not numerically identical to my sperm and the sperm itself is not a potentially rational person either), rather the opposite. That harm is done to some kind of spiritual good (preservation of mankind) has already been refuted. The same applies to contraceptive sex. In addition,
preventing a new life from beginning is not the same as willing an existing life to end. Contracepting cannot be contra-life unless there is some life towards which it is ‘contra’. Anyway, from the wider perspective of current Catholic doctrine, infertility does not render heterosexual intercourse ‘contra-life’[.] (Murray, T. M. - Thinking Straight About Being Gay)
Or does the Thomist only refer to the intention of action and not to a harmful order? "Contrary to" could then mean acting for the sake of actively and deliberately frustrating (or preventing, avoiding, impeding, destroying) the realization of a natural end of one’s activated faculties.

Uses and acts would mean in "contrary to" when my intention is exclusively to prevent the natural end of my used faculty from achieving its realization. One must not forget that each faculty, once activated, is focused on its end, which it needs help to achieve. Every natural faculty once activated or used, is like a man lying on the street in urgent need of help. The unavoidable moral goal is clear to the helper. He cannot ignore it. And the injured person cannot help himself. 

With pure and morally neutral "other than" acts or uses the intention is not yet clear at all. It could contain much, perhaps everything, except the intention I just mentioned for the bad case.

But the following oddities could arise here: Chewing gum, playing soccer and walking on your hands and so on for the sake of preventing (frustrating or destroying; you can call it as harsh as you want, it's always the same) the realization of the end (or purpose or goal) of the corresponding faculties would be evil. Next time I chew gum, I can tell myself I’m just chewing for active frustration. That would have to be a major moral offense.

As for sex, who actually says "I want to actively frustrate" in their head during sexual intercourse? There are no such intentions. My real intention, as an example, is innocent pure fun, for which in my opinion the possibility of STDs and the responsibility for a possible new life is excluded.

But then everything would come down to what would be obviously absurd: You don't have to deliberately head for the actual end, but you are not allowed to deliberately bypass it. The Thomist says, in a way, that my intention does not have to be to realize the purposeful and proper arrangement (end) of my natural parts when I use them. Only I am not allowed to undermine deliberately the realization of the actual end of my used parts by arranging them in a specific way. This seems inconsistent.

A problem arises when I activate and use a natural faculty for something other than the natural end and know in advance exactly what its functions are. So if I consciously strive for something other than the natural end with this natural and activated faculty, this inevitably leads to the impossibility of realizing the genuine natural end. And now comes the crux: I am well aware of this fact, as I know all the functions. This means that if I strive for something else, I know automatically and at the same time that I am actively thwarting the realization of the true end. Because once some faculty is activated, it has a teleological orientation in need of help. There’s no way around it.

To do something for the sake of frustration of the proper end (contrary to) cannot happen apart from seeking a deviant one (other than). The complete knowledge of a natural faculty, when misused, will always imply the intention of the "contrary to" case. Knowledge is actually presupposed in the moral discussion (ethics always begins its discussion with those who know, it does not assume innocent non-knowers), whereby the artificial distinction between "contrary to" and "other than" could become obsolete.

If I said the following "To do something for the sake of frustration of the proper end cannot happen apart from seeking a deviant one" it sounds a bit paradoxical. This is about the old problem of whether you can do something for the sake of evil, that is, evil for the sake of evil only. I think even for Aquinas this is not quite possible, because according to him reason tries to recognize the true essence of things and the will acts only accordingly.

The active and deliberate prevention of the realization of a natural end is usually only a means to achieve another probably rather innocent (but according to natural lawyers unnatural) end. If I now know about the only true purpose of sexuality, namely reproduction, then we would also have the double-intention affair. The fact that I want to exclude, in particular, procreation, belongs – as a case of not wanting to include the realization of the true end – to that evil intention. The "evil" intention runs parallel and subordinated to the innocent one. You can say the same thing about chewing gum. I want mere fun, but achieving this mere fun is only possible by preventing the realization of a natural purpose, which would be evil.

Both acts or uses discussed are always cases of "contrary to" when the absolute knowledge of human nature is given, and which knowledge coincidentally matches with that of the Thomists. Or both are always cases of "other than", because we can never be sure how many main functions there are, and most people act innocently and don’t think when they act in such technical terms or in biological functionalities. The distinction, therefore, does not seem to be tenable.

What else could there be that would lead me to the evil intention: Only the knowledge of all (natural) functionalities and ends and of all circumstances. Through the knowledge I would perhaps undermine the realization of the true purpose in the following way: I would achieve my "other than" goal only by the means of the "contrary to" of the known true purpose. How else would I be able to achieve the "other than" goal? And the "other than" goal is not the true goal. Approaching the former goal is like moving away from the latter. The knowledge then gives the entire thing a deliberate character. That all this, in turn, leads to absurdities has already been demonstrated by the example of chewing gum.

In both cases, a completely different end is actively and knowingly targeted (like keeping indigestible and undecomposable gum in your mouth), which means that the realization of the proper end (to spit out the gum) is actively and knowingly frustrated. This happens every time most intentionally and the non-achievement of the end is completely foreseen. All this is actively facilitated.

The question is also from whom actually the judgment comes whether something corresponds to the "other than" or the "contrary to". From yourself or a third person? If one person judges another, and the judging person knows that the judged person has no idea of any purpose, then it is clear that the judging person can only identify an "other than". From the point of view of the acting person, there is neither an "other than" nor a "contrary to". It is probably impossible, through experience (from whatever point of view: 1st person or 3rd person), to identify a single case where the intention of an act consisted solely in a "contrary to". It can never be excluded with certainty that really no secret "other than" motive has constituted the determining intention. This consideration is freely based on Kant. Moreover, this consideration is also based on the idea that we can know exactly what human nature consists of. But we can never know that with absolute certainty, let alone that one true everlasting universal nature exists.

For the determination of "other than" and "contrary to", the Thomist should commit himself to one meaning. Is the criterion for a bad moral act solely the intention or solely (the realization of) a harmful order? Either or!

Let's assume that a man wants to use a condom to avoid possible procreation during sex. This is a clear bad intention for natural lawyers. But what if the intention is not successful, because the condom slips out by coincidental circumstances and bad luck so that unhindered ejaculation in the vagina is given? So we would have morally good action in this?

What if we move away from the aspect of intention and return to the aspect of harm? Then one could say that natural law has only negative commandments, that is, only commandments that contain "you shall not do this and that". No positive rules. But this cannot be convincingly stated by the Thomists. In such a case one must no longer act according to nature, i.e. according to a natural faculty, which would distinguish the morally good. The morally good would only be about not acting contrary to nature. One can now refer to the following: I have the impression that almost all organs and faculties allow the use of the "other than" option except for sexuality ("other than" would be equated with "contrary to" and would mean ejaculation outside the vagina). So there's just a rule not to ejaculate outside the vagina. If there were a positive law ordering ejaculation in the vagina, this pattern could be applied to all natural abilities and the "other than" criterion would be abolished. Because then I would always have to try to reach the natural end. Does ethics make any sense out of only negative precepts? I don't think so. I could question the rule of not being allowed to ejaculate outside the vagina, so I should definitely be given a positive rule. The function of the natural faculty would give the appropriate precept (ejaculation in the vagina) and so we would again move in circles.

Or did I misunderstand something? Then the distinction the Thomists make between "other than" and "contrary to" seems to be very vague and unclear. "Other than", in my opinion, can only have three meanings. Either it refers to another function of a faculty that has not been mentioned before. Or it refers to another way (other means) and not to the conventional or natural one to achieve the goal of the function. Or one understands by it the inappropriate and perverted activation of a faculty, which leaves, however, no damage to the function, the faculty, or the whole human organism (this would then be a matter of interpretation). In the latter case, the function does not achieve its goal because the faculty has been perverted. In any case, these proposed meanings must be related to a deliberately activated faculty, otherwise "other than" would be a meaningless concept. When I consciously activate a faculty, there is only one either-or: Either I try to achieve the goal of its function to the best of my knowledge and conscience, or I do not allow the goal to be achieved. As soon as I act in some way, I inevitably use natural faculties and organs that I have. All organs and faculties have functions, otherwise, they would not be organs and faculties. Now I can respect these functions or not, which means that I can or cannot help these functions to reach their goal properly. And consciously not helping means consciously preventing. If I do not know of any function, of any faculty, of any corresponding goal, then I do nothing that can somehow be understood as “other than”. "Contrary to" can only mean perverting a natural faculty, i.e. deliberately preventing it from achieving the goal of its function. This would mean involving damage if the third definition of "other than" came into force.

One could perhaps classify the "contrary to" possibilities in this way. Serious transgressions: I deliberately pervert a natural process into which I have deliberately entered that takes place only voluntarily and willfully. Minor offenses: I deliberately pervert a natural process that takes place on an ongoing and involuntary basis. However, this distinction would not be reasonable, as an artificial stopping of the heartbeat would only be a minor matter and the chewing of pure caoutchouc gum for many many slightly unhealthy hours would be a big issue. In the end, one cannot avoid to arbitrarily evaluate each human function individually and classify it according to importance.

In order to make the distinction more understandable, one would need to describe it better and more clearly using razor-sharp definitions and also, compare two faculties with each other, and say in advance which definitive functions they have (regardless of whether one can argue about these).

Sex with sterile people

Is there actually a real difference between sterile sex and contraceptive sex? I do not think so, and the Thomists make a distinction possible only through certain additional criteria, none of which I believe are tenable.

I will first present two scenarios to make my point clear.

First, a man deliberately seeks sterile women in order to satisfy his sexual appetite without using a condom and without the possibility of pregnancy. This man should not act any differently in the natural law sense than someone who goes on a hunt for fertile women using condoms.

Secondly, a man is in marriage but does not want any more children. Thus, he stopped having sex with his wife. If at all, he only tries to use the temporarily sterile natural phases of his wife for sex. Now the man learns that his wife will not be able to conceive for quite a while due to medical intervention or treatment. This is one hundred percent certain. The man is now very happy, for he can have a lot of sex without running the risk of having another child, which he could not care for properly anyway at the moment due to financial reasons. And he doesn't have to look annoyed at the fertility calendar anymore.

In both cases the intention is the same, there is the same sexual external order of the body parts, and also the negative result is identical. The only difference is marriage in the second case. If this legitimizing criterion of marriage is not tenable, natural law in the realm of sex collapses like little dominoes step by step.

So the purely monogamous marriage and its indissolubility must first be convincingly justified in a naturalistic way. The adherence to an indissoluble monogamous marriage, although the man could perhaps still provide for many women without problems and raise many children in an exemplary manner (maybe his present children have all grown up and become independent), is for me rationally incomprehensible. Therefore one can confidently say here: One must, for example, endow the nature of men with abundant Western cultural assets in order to be able to justify the monogamous marriage or even its indissolubility. Non-serial monogamy and anti-polygamy do not belong to human nature from the point of view of evolutionary psychologists. Good Bertrand Russel also goes into it in detail:
Take, for example, the indissolubility of marriage. This is advocated on the ground that the father is useful in the education of the children, (a) because he is more rational than the mother, (b) because, being stronger, he is better able to inflict physical punishment. A modern educator might retort (a) that there is no reason to suppose men in general more rational than women, (b) that the sort of punishment that requires great physical strength is not desirable in education. He might go on to point out that fathers, in the modern world, have scarcely any part in education. But no follower of St Thomas would, on that account, cease to believe in lifelong monogamy, because the real grounds of belief are not those which are alleged. (Russell, Bertrand. History of Western Philosophy)
Without the marriage criterion, any act of sex with a sterile person who is known to be irreparably sterile is a perverting act, as in the case of contraception use. If I look at natural law purely naturalistically, it makes little sense to be allowed to have sex, even though one knows for certain that fertilization cannot take place. Whether I just let my semen out on the floor or in a sterile person, it makes no difference (ejaculation in the vagina cannot suddenly become the main goal, which is still procreation. The Thomists are always disingenuously jumping back and forth here). Natural law must clearly prohibit sexual intercourse when one knows that the other is demonstrably infertile since the maintenance of the human species is supposed to be decisive (but the Church does not want this prohibition since it contains absurd implications).

If a man has sex with a woman's corpse (who happens to be his late wife), whereby the ejaculation takes place completely catholically, there should still be a perversion according to Thomists. If only the woman's procreative faculty is dead, then said ejaculation is fine, but only as long as the woman is also his conjugal wife. 

Here I will give further examples. I could use a sterile woman, of course in consensual sex, to realize my intention, namely the intention to let my semen out for pure pleasure without the possibility of pregnancy. So my goal is to intentionally activate my sexual faculty while preventing the realization of the natural end of this faculty. But I could also use a condom when having sex with a fertile woman so that the same intention can be realized. There is really no difference between these two actions in terms of their intention. I don’t necessarily have to manipulate anything about myself, I could also ask a woman to use a diaphragm. 

When having sex with a sterile woman, the Thomist surely won’t want to persuade me that I should imagine the possibility of miraculous fertilization as belonging to my intention. I will surely have some intention, and perhaps that will only be pure love and lust so that I would have sheer moral luck not to use contraceptives. If the woman had not been sterile, I would have had to use contraceptives to grant my lust.

Since sex is a process between two people, I have to consider the process as a whole. I have to see if the other part of the whole is also in accordance with the natural end of the whole, which I do not think is the case with sterility on the other part, even if no one had any influence on it, and which is also not the case when it comes to contraception on the other part. If I know that the circumstances cannot possibly lead to the realization of the purpose of the whole and still, get involved in the activation of the whole, one could interpret it in such a way that I consciously and actively want to undermine the realization of the purpose by merely entering deliberately into the process. This is indeed possible.

If that is not possible, the Thomist might say the following (Feser does say this): The malignancy or goodness (or moral neutrality) of sterile sex depends on whether both parties love each other, are married and pretend to be procreating a child when having sex. But these are criteria that are highly problematic and still need to be justified.

Permanent sterile (people without reproductive organs, i.e. without testicles in men and without uterus and ovaries in women) couples have requirements when they want to have sex according to Thomists. On the one hand, the sexual act must resemble externally the sexual act between completely fertile couples. On the other hand, a union filled with love (affection) must be sought. If these requirements turn out to be untenable, then the Thomist would have to admit that sex between sterile people is a perverted act of sex (as a whole). After all, the couple would see their mutual sterility as a means of contraception. They couldn't see it any other way.

Thus sterile sex must end with some form of ejaculation in the vagina (perhaps only with an imaginary form, accompanied by dry orgasms when ejaculation is no longer possible). Anything else would be sinful. This is absurd. Why should the couple pretend? That makes no sense. If certain organs or natural abilities are missing, there can be no more purposes for them. If I don’t have eyes anymore, I’m not obliged to pretend I can see with eyes. It would only be a superstitious adherence to a mere external form or order of things. The commitment to doing as if (despite loss and incompleteness of functions, so that certain final causes no longer exist) makes naturalistically no sense at all. I would like to have examples outside the realm of sex. Breastfeeding of infants, for example. If there are no milk glands, then the woman does not have to pretend to give her children milk. This would be of no use to the baby. Someone without legs does not have to fake walking with legs. If I'm insane, do I have to pretend I'm sane? If a natural faculty is destroyed by the absence of an essential part, then that faculty ceases to be a faculty and ceases to have a telos. If I can no longer act according to this telos, then I can no longer act contrary to it. If I can't achieve the end, I can't pervert it. Andrew Koppelman also sees the problem:
[I]t’s far from clear in what sense, that has any moral weight, the genital organ of a sterile man can properly and precisely be called a reproductive organ. It is not fit for reproduction. (Andrew Koppelman - The Gay Rights Question)
From here natural law becomes spiritual and supernaturalistic. The Thomist apparently imagines that during sex the Platonic idea of sex would float over you and that you should get as close as possible to it. (How should a completely blind person be able to approach the idea of seeing? He can only abandon this idea completely in order to move on to another idea of capturing the outside world, such as sound-orientation.)

The prerequisites mentioned are therefore to be understood spiritually. With that, they are purely a matter of faith. Sex between man and woman is about an "ideational community in which they participate" (Andrew Koppelman). It is no longer about the realization of a naturalistic form, which, as we have seen, would be absurd in the case of sterility, but about the realization of a supernatural form.

So there are many basic (spiritual) prerequisites in natural law. Such as the sudden obligation to an inner inclination (here love) in a certain context (sterility). Apart from the fact that context instantly plays a role, where could such a duty arise outside of sexuality? If I am not capable of love at all, does another obligation to a different inner disposition strike me?

Within the aforementioned conditions, sterile sex becomes a case of "other than". Without them, it becomes "contrary to". These prerequisites have nothing to do with the general perverted faculty argument. They are simply thrown in from the side without having a proper place in the ethical system. One can assert a lot. But how are these prerequisites deduced, justified, and systematically located? If these additional prerequisites are poorly justified, the scope of natural law in the field of sexuality no longer seems plausible.

The church also sees only the spiritual meaning of the sexual act:
As Humanae vitae has made clear, the true disorder in a contraceptive act is not "frustration of nature" but disruption of the symbolic nexus of love and fertility.

If some action is condemned, it is not because it violates animal biology, but because it is contrary to something in the special relation of two persons united by marriage encountering each other in conjugal intercourse.
(John T. Noonan, Jr. - Contraception)
Everything that is discussed from now on ignores the spiritual preconditions.

If I know that my wife is sterile forever, then I can hardly have the intention to impregnate her. That would border on insanity (or the supernaturalistic belief in miracles comes into play).

In the case of sterility, the intention suddenly no longer plays a role. Why is that so? Only an outer order should be maintained. So we have again the problem that sometimes the intention is important, another time the right outer arrangement of natural things.

Various scenarios that could make natural law questionable:

Scenario 1 (all following scenarios deal with the morality of the man independent of Christian self-understanding): Woman 1 just took the pill. Woman 2 is completely sterile through no fault of her own (complete sterility was impossible to detect at Aquinas’ time). The man knows about the impossibility of fertilization in both women, but he does not know for sure whether woman 2 really became sterile without her fault. He will never be able to know all this one hundred percent. He then gets involved in sex with them out of love. But sex with woman 1 and sex with woman 2 lead to the same result: procreation is impossible in principle, i.e. the man has deliberately let the semen come out in such a way that procreation cannot follow. He did not manipulate his own reproductive ability. But he deliberately perverted the sexual process as a whole. Preliminary thesis: If sex with woman 2 is allowed, then he must also be allowed with woman 1. Because: Where is the difference in relation to the sex process as a whole? Morally it should depend on the fact that I pervert the process as a whole and not how I do it in detail. Perversion itself, like murder itself, is evil. And not how I pervert or how I murder.

Scenario 2: Woman 1 is completely sterile by no fault of her own. Woman 2 does not use anything that could somehow prevent pregnancy. Woman 3 is using calendar contraception. The man has sex with the three women (though in each case thought in a different parallel universe). With woman 2 he puts a condom on before. Here the man perverts his reproductive organ, which also results in perverting the sex process in its entirety, as in the first scenario with woman 1 and also with woman 2. When having sex with woman 1 in the second scenario, the man sees the woman’s non-existent reproductive capacity as a kind of permanent birth control device. No condom is used with woman 3 because she is allergic to it, but the probability of fertilization due to calendar contraception is just as low.

What follows from everything: There is no difference between having sex with woman 1, woman 2, and woman 3. Internally the man does not want children in all cases, externally he goes into a process which will never reach his goal (a pregnancy with woman 2 and 3 has however still a small probability, differently than with woman 1). If the preliminary thesis from scenario 1 is correct, i.e. the woman can use contraceptives on her own without the man getting into moral difficulties. Then why can’t the man use them himself? Otherwise, there would be a moral and legal asymmetry between the man and the woman. In all three cases, the man is deliberately involved in a process where achieving the goal is impossible or very unlikely. In all cases, the whole process is perverted. Because: If I understand the ultimate goal of sex (fertilization) as an absolute moral imperative, I should not consciously make the achievement of that ultimate goal less likely.

New scenario: A: The woman only takes the contraceptive pill for health reasons. In fact, she wishes for a child. B: The woman checks her fertility with all possible natural means and always communicates the results to her husband. She also secretly wishes to have a child. C: The woman lost her uterus guiltlessly and is therefore sterile forever. In these cases A, B, and C the man does not want children when having sex with the women, that is his intention; he also does not want to use a condom, whereby he could manipulate his sexual organ. The man just wants to have fun and knows about the situation of these women. D: The woman is already pregnant, so she can’t get pregnant again. In this case, the man also wants only fun and he is grateful that further procreation is biologically impossible. The attitude of the man is nevertheless to be understood in such a way that he does not want to have another child with the current sexual act. The non-procreative intention and the non-procreative result (or the non-procreative order of the sexual act) are identical in all cases.

The argument of the manipulation of one’s own sex organ is only secondary to that argument of the actual intention not to procreate by engaging in a sexual act without the possibility of procreation. Because in the case of A, the woman could take the pill for contraceptive reasons, while the man leaves his sexual organ undisturbed and unaffected in this respect. I don’t think the Thomist would want to morally justify case A. He might say the man should ask the woman to stop taking the pill before they can have sex again. If the two have infertile sex, after all, the Thomist has to say that the sex act as a whole, which always includes two people, is disturbed and perverted both from the point of view of the man and the woman.

The Thomist secretly admits that the important point is whether the sexual act as a whole is perverted when he talks about permanently sterile persons. Because they could only have sex under certain conditions. If these conditions cease to exist, the Thomist would admit that there is a perverting of the whole sexual act that takes place among the sterile and infertile people. The argument of the whole can be applied here to all cases, the manipulation of one’s own organ is directed only to the thwarting of the whole. But the manipulation is not obligatory, as one could see in case A. If the intentional thwarting of the whole is permitted, then it is permitted in whatever ways. The Thomist cannot then claim that the intention suddenly plays no role at all in the sterile cases A, B, C, and D. It is unclear what the Thomist has to say about the so-called natural contraception permitted by the Church. This type of contraception definitely implies the intention to undermine the natural end of sex. It is quite certain that in the case of D the Thomist would object if the man used a condom out of a sudden hypochondriac but totally unfounded fear of an STD. Prohibiting the use of condoms in cases of natural impossible conception would be pure superstition. There could be cases where the man with a sterile woman does not want to have a child regardless of whether he uses a condom during sex or not. Whatever you want to say, the intention is the same. The man just wants to have fun and definitely not a child. He just wants/intends to unload his seeds out of pure lust, without the possibility of children arising. He wouldn’t care about the condom at all, because he knows it doesn’t add anything to his intention.

What about people who intentionally let themselves be artificially sterilized once in their lives? And they deeply regret this irreversible intervention? Are they forever excluded from sex? That would be ridiculous. One could only say that they have sinned once. But you couldn’t blame them for their sexual actions if they were carried out according to the supposed natural standards.

Masturbation

In principle, the Thomist would have to admit that masturbation can be morally neutral. Criteria for moral neutrality in this case: You don’t have to consciously intend to realize the natural end of your sexual (ejaculation) faculty. You can use your sexual (ejaculation) faculty when you know its natural end won’t, in fact, be achieved. You can also use this (ejaculation) faculty and foresee that the end of this natural faculty will not be realized. And most importantly: My intention is not frustration, but fun, health, relaxation, perhaps even love in thought. The Thomist would perhaps mention here the unifying function, but as I said, my intention is not to frustrate any function, hence also not the unitive one. So we have here a clear case of "other than". The good effect intended outweighs the frustrating effect on the generative faculty. 

And masturbation surely has many goods:
3) masturbation can relieve sexual tension (Francoeur, 1991);
4) masturbation can be used as a form of safe sex (Davidson and Moore, 1994);
5) masturbation can help people learn about their sexual likes and dislikes (Phipps, 1977);
6) masturbation can be used to help treat certain sexual dysfunctions, such as anorgasmia in women and delayed ejaculation in men (Rowan, 2000);
7) married women who masturbate report greater marital and sexual satisfaction than women who do not masturbate (Hurlbert and Whittaker, 1991); and
8) masturbation can result in a sense of well-being and can even result in higher self-esteem (Hurlbert and Whittaker, 1991). (CHRISTOPHER ARROYO - Natural Goodness, Sex, and the Perverted Faculty Argument)
Psychologically, there is no difference between masturbation and sex with a contraceptive user who appears as a masturbation device, even when mutual short and little love is involved. (In that case, one could say, I would’ve just known shortly before sex that she was using birth control. Does the intention of the other person then influence the morality of my own intention? Morally, only each person is considered individually and I have not asked the other woman to take contraceptives, nor do I know the exact reason why she is taking contraceptives. Would it be necessary to ask about all this before having sex, although any answer would be uncertain? For simplicity’s sake, let’s say the person, in this case, took the pill for purely hormonal reasons, that is, for health reasons. I have heard before that this happens.) Both cases are "other than" because the same intention is given. (Would I have to ask first why the other person is sterile, namely whether the sterility is self-inflicted or not? But I will never know about the true intention, no matter what the person says, it could be the other way around because the other person might not know about her true intention either. It could have been a completely innocent intention.) What would morally classify these cases as bad. Apparently only the intention to frustrate. But this intention seems to be very difficult to implement and to be done. And the intention to avoid stress with a possible baby (that means to achieve relaxation, fun, and well-being) may not be the same as the intention to actively prevent the realization of the natural end (that is, to frustrate this end). Thus, the use of any form of birth control could be morally neutral by all participants, since those contraceptives only serve the "other than" purpose.

Final words and quotations (on suicide and bestiality)

The Thomistic ethics of natural law is no longer appropriate to the times and their doctrines are no longer perceived as weighty. Personally, I will never be able to morally condemn lived out homosexuality or suicide, two great sins in this ethics because I cannot at all see and feel the absolute reprehensibility attributed to them by the natural law scholars. Whoever is a natural lawyer, however, must hold up the condemnation in an obtrusive manner and thus very probably isolate himself socially. Here are some opinions about suicide that I can only agree with: For Schopenhauer, it is quite obvious that
there is nothing in the world to which everyone has such an indisputable right as his own person and life. (Schopenhauer - §157 On suicide Parerga and Paralipomena: Volume 2 translated and edited by Adrian Del Caro and Christopher Janaway with an introduction by Christopher Janaway)
Because:
Duties of right towards ourselves are impossible, because of the self-evident fundamental principle ‘No injury is done to someone who wills it’: for since what I do is at all times what I will, what happens to me from myself too is always only what I will, and consequently never a wrong. (Arthur Schopenhauer – The Two Fundamental Problems of Ethics)
Then:
Suicide, as mentioned, is even counted as a crime, and so it carries with it an ignominious burial and confiscation of one’s estate, which is why the jury almost always renders a verdict of insanity. Let us try for the time being to allow our moral feelings to decide on this matter, and compare the impression on us of the news that an acquaintance has committed a crime, say murder, cruelty, fraud or theft, with the news of his voluntary death. Whereas the former occasions lively indignation, supreme resentment, and calls for punishment or revenge, the latter arouses sorrow and compassion often mixed with a certain admiration for his courage, rather than the moral disapproval which accompanies a bad action. Who has not had acquaintances, friends, relatives who have voluntarily departed this world? And everyone is supposed to think of them with revulsion, as criminals? I say no and no again! (Schopenhauer §157 On suicide Parerga and Paralipomena: Volume 2 translated and edited by Adrian Del Caro and Christopher Janaway with an introduction by Christopher Janaway)
Finally:
Is Hamlet’s monologue the meditation of a crime? [What little mind would think that?][...] But the objections to suicide which are proposed by the monotheistic, i.e., Jewish religions and the philosophers who cosy up to them are weak, easily refuted sophisms. (Schopenhauer §157 On suicide Parerga and Paralipomena: Volume 2 translated and edited by Adrian Del Caro and Christopher Janaway with an introduction by Christopher Janaway)
Now we listen to Nietzsche:
The two greatest judicial murders in the world's history are [Jesus and Socrates], to speak without exaggeration, concealed and well-concealed suicide. In both cases a man willed to die, and in both cases he let his breast be pierced by the sword in the hand of human injustice. (Friedrich Nietzsche - Miscellaneous Maxims and opinions in Human, All Too Human II)
And:
Which is more reasonable, to stop the machine when the works have done the task demanded of them, or to let it run on until it stands still of its own accord—in other words, is destroyed? Is not the latter a waste of the cost of upkeep, a misuse of the strength and care of those who serve? Are men not here throwing away that which would be sorely needed elsewhere? Is not a kind of contempt of the machines propagated, in that many of them are so uselessly tended and kept up?—I am speaking of involuntary (natural) and voluntary (reasonable) death. Natural death is independent of all reason and is really an irrational death, in which the pitiable substance of the shell determines how long the kernel is to exist or not; in which, accordingly, the stunted, diseased and dull-witted jailer is lord, and indicates the moment at which his distinguished prisoner shall die. Natural death is the suicide of nature—in other words, the annihilation of the most rational being through the most irrational element that is attached thereto. Only through religious illumination can the reverse appear; for then, as is equitable, the higher reason (God) issues its orders, which the lower reason has to obey. Outside religious thought natural death is not worth glorifying. The wise dispensation and disposal of death belongs to that now quite incomprehensible and immoral-sounding morality of the future, the dawn of which it will be an ineffable delight to behold. (Friedrich Nietzsche - Miscellaneous Maxims and opinions in Human, All Too Human II)
At last:
Disregarding the demands made by religion one might well ask: why should it be more laudable for an old man who senses the decline of his powers to await his slow exhaustion and dissolution than in full consciousness to set himself a limit? Suicide is in this case a wholly natural obvious action, which as a victory for reason ought fairly to awaken reverence: and did awaken it in those ages when the heads of Greek philosophy and the most upright Roman patriots were accustomed to die by suicide. Conversely, the compulsion to prolong life from day to day, anxiously consulting doctors and accepting the most painful, humiliating conditions, without the strength to come nearer the actual goal of one's life: this is far less worthy of respect. Religions provide abundant excuses to escape the need to kill oneself: this is how they insinuate themselves with those who are in love with life. (Friedrich Nietzsche - Human, All Too Human: A Book for Free Spirits)
Fritz Mauthner also has something interesting to say on the subject:
The question of whether suicide is allowed or not is so highly stupid that any attempt to answer it must become stupid. One could ask with equal rights whether an oak tree is allowed. And if suicide is considered something rare, a case against the rule, one might ask with equal rights: whether a calf with six feet is allowed. And if one opposes me by saying that suicide is not a natural product, but a human act, I ask, first of all, whether human acts do not fall under the expressions of living nature, and then I ask, whether reproduction, whether chastity, whether wine enjoyment, whether abstinence is permitted. By whom allowed? By God? From the law? A different question would be serious: how is suicide possible? Which motive can be stronger than the instinct of self-preservation, which in the mass of people coincides with life, ego-feeling or consciousness? Man is capable of suicide because more conscious motives can become stronger than the instinct of self-preservation. But it is not entirely correct to deny animals the ability to commit suicide. Let us first take an extreme and invented case. A cat, which would inevitably drown in water, clings to the stones of a vertical bank wall. People or ghosts with glowing iron bars approach her from all sides. The glowing poles are so close to each other that the cat can neither slip above nor beside them, nor between them. Now the people or ghosts bring the embers nearer and nearer to the cat. The cat is not pushed, not burned. She is only pushed closer and closer to the edge of the wall. And who can say whether she let go at the last moment with free and conscious will or not. One will remit me a utility application to the human suicide. But also the indirect suicide of animals could be brought under the scheme of this cat: the bird, which lures the enemy from the nest in mourning for its young and thus falls victim to it; the captive animal, which in fear of death pulls out a limb and perishes as a cripple; the animal, which in the agony of hunger consumes food, which brings about death. As in the cat of the scheme, the stronger instinct is that of escaping a highly positive pain at any risk. Now it is strange that man's nature, without any act of will and without consciousness, has formed a means of protection against excessive pain. If an unbearable pain storms into man, a pain that the life force cannot bear, nature protects itself by sudden death or by apparent death, which in this case is called swoon. Is death as a result of illness - from suffocation, as is often the case with diphtheria, from a kind of poisoning, as is the case with cholera, etc. - not also an escape from unbearable damage? Is not the natural death, which alone deserves this name, the old age death, also a resignation of the last life force, which starves rather than is content with miserable surrogates of the metabolism? So are there not transitions between suicide and death? And because the so-called suicide is also not an unnatural death, because it always happens naturally - since man always belongs to nature, both in life and in death - I am therefore inclined to prefer the new, not quite perfectly formed expression free death [Freitod] - not yet booked in D. W. - to the words suicide [self murder], which are reminiscent of the language of criminal law and age. [...] Free death [Freitod] reminds me, like a flight of steps, a free statue, of something that leads to the outside, that grants freedom. (my own translation from German, Fritz Mauthner - Wörterbuch der Philosophie, dictionary of philosophy)
So there's no reason to morally condemn suicide. Nevertheless, suicide should always be prevented, as it only occurs out of a temporary phase of despair or depression. Hume's thoughts on suicide are also worth reading: 


But what about bestiality, that is, sex with animals? After all, we no longer need to talk about homosexuality which today has rightfully become normality. However, sex with animals is very perverse, but not morally evil. When I have such an opinion, I do not need a natural law, let alone a Thomistic natural law. Bestiality is relatively high on the perversity scale, higher than, for example, lived out urophilia. Nevertheless, as long as no one is used merely as a means, everything on this scale remains free of moral reprehension. But I also say that bestiality is a case for the psychologist. Anyone who has sex with animals should go to mental treatment. I'm assuming such a person isn't very happy. Isolation from loved ones and possible spread of diseases speak, of course, also against bestiality.

Before I come back to bestiality, the following things have to be said. Whether a sexual activity is natural or perverted does not depend on what organs are used or where they are put, but on the character of the psychology of the sexual encounter. Then the following must be considered:
Sexual behavior differs from other behavior by virtue of its unique feelings and emotions and its unique ability to create shared intimacy. (Moulton, Janice: Sexual Behavior: Another Position. In: The Philosophy of Sex Contemporary Readings. Fourth Edition. Edited by Alan Soble 2002)
Nagel analyzes
sexual desire as a “complex system of superimposed mutual perceptions” He claims that sexual relations that do not fit his account are incomplete and, consequently, perversions. Solomon claims that sexual behavior should be analyzed in terms of goals rather than feelings. He maintains that “the end of this desire is interpersonal communication” and not enjoyment. (Moulton, Janice: Sexual Behavior: Another Position. In: The Philosophy of Sex Contemporary Readings. Fourth Edition. Edited by Alan Soble 2002)
Thomas Nagel then
characterizes sexual perversion as a “truncated or incomplete version” of sexual arousal, rather than as some deviation from a standard of subsequent physical interaction. (Moulton, Janice: Sexual Behavior: Another Position. In: The Philosophy of Sex Contemporary Readings. Fourth Edition. Edited by Alan Soble 2002)
Robert Solomon, for example,
argues that the aim of sexual behavior is to communicate one’s attitudes and feelings, to express oneself, and further, that such selfexpression is made less effective by aiming at enjoyment[.] (Moulton, Janice: Sexual Behavior: Another Position. In: The Philosophy of Sex Contemporary Readings. Fourth Edition. Edited by Alan Soble 2002)
Bertrand Russell brings the moral aspect to the point:
Morality in sexual relations, when it is free from superstition, consists essentially of respect for the other person, and unwillingness to use that person solely as a means of personal gratification without regard to his or her desires. (Bertrand Russell - Marriage and Morals)
Finally, back to bestiality:
[T]o the extent that someone engages in bestiality, she will find it harder to retain a grip on her identity as a full member of our community, and we will find it harder to admit her to full membership. It is because bestiality is identity-threatening in this way, I submit, that we suspect those people who do decide to cross this boundary of psychological illness. They have chosen to remove themselves from our community, as it currently defines itself. Crossing the species boundary is a significant act, at least for us, here and now, as we currently define ourselves. (Neil Levy - What (if Anything) Is Wrong with Bestiality? In: JOURNAL of SOCIAL PHILOSOPHY, Vol. 34 No. 3, Fall 2003, 444–456.)
And:
Thus, though there is nothing immoral about bestiality, it might nevertheless be irrational for us to cross this boundary. (Neil Levy - What (if Anything) Is Wrong with Bestiality? 
As a non-natural lawyer, I am nevertheless allowed to use knowledge about nature to draw certain conclusions, I am just not allowed to build any morals on it. I broadly agree with these quotes and it should be clear why bestiality is not good for a person who wishes for shared emotional depth and deep intimacy in his innermost being (the intense companionship of happy mutual love), who needs affectioned and existential communication (verbal, nonverbal, or via touches) with another human being. It's a little like masturbating. The act of masturbation is just a cheap replacement. Because the masturbating person wants to have sex with a real person. This is clearly expressed in his/her fantasy.

III.2) Critical literature that must be read if you want to delve into this thankless subject and wash your head clean

A few web recommendations:

Part 4.3: Egregiously Applied Ethics: Pray the Gay Away!

Part 4.4: Egregiously Applied Ethics: Praying the Gay Away, With Friends!

https://gunlord500.wordpress.com/2016/09/30/against-aquinas-an-in-depth-critique-of-natural-law-ethics-theology-and-metaphysics/

A DEFENSE OF THE SEXUAL REVOLUTIONARY

https://gunlord500.wordpress.com/2019/08/06/a-defense-of-the-sexual-revolutionary/

FUN WITH FINAL CAUSALITY

https://gunlord500.wordpress.com/2018/02/04/fun-with-final-causality/

The Perverted Faculty Argument: A reply to Edward Feser.

https://angramainyusblog.blogspot.com/2019/01/the-perverted-faculty-argument-reply-to.html

Natural Law, or Don’t Put a Rubber on Your Willy

http://theanarchistlibrary.org/library/robert-anton-wilson-natural-law-or-don-t-put-a-rubber-on-your-willy

The Myth of “Natural Law”

https://theanarchistlibrary.org/library/iain-macsaorsa-the-myth-of-natural-law

Philosophy of Sexuality

https://www.iep.utm.edu/sexualit/

Requiem for final causes

https://mappingignorance.org/2019/03/04/requiem-for-final-causes/

A critique of natural law theory

http://www.newcivilisation.com/home/ideas-philosophy/a-critique-of-natural-law-theory/

Sex and Sexuality: Criticising Natural Law Theory

https://www.patheos.com/blogs/tippling/2017/06/08/sex-sexuality-criticising-natural-law-theory/

The Universe Doesn’t Care About Your ‘Purpose

https://www.nytimes.com/2017/07/31/opinion/the-universe-doesnt-care-about-your-purpose.html

IS, OUGHT, AND NATURE'S LAWS

https://www.firstthings.com/article/2013/03/is-ought-and-natures-laws

Criticizing the idea of potential and actuality in natural law philosophy


Critical papers:

CAPALDI, NICHOLAS: USING NATURAL LAW TO GUIDE PUBLIC MORALITY - The Blind Leading the Deaf. In: THE DEATH OF METAPHYSICS; THE DEATH OF CULTURE - Epistemology, Metaphysics, and Morality. Edited 2005 by Mark J. Cherry

Donagan, Alan: THE SCHOLASTIC THEORY OF MORAL LAW IN THE MODERN WORLD. In: Aquinas – A Collection of Critical Essays edited 1969 by Anthony Kenny

DUBOIS, JAMES M .: Is Anesthesia Intrinsically Wrong? On Moral Absolutes and Natural Law Methodology. In: Christian Bioethics , 14(2) , 206 – 216, 2008

DUBOIS, JAMES M.: How much Guidance can a Secular Natural Law Ethic Offer? A Study of Basic Human Goods in Ethical Decision-Making. In: THE DEATH OF METAPHYSICS; THE DEATH OF CULTURE - Epistemology, Metaphysics, and Morality. Edited 2005 by Mark J. Cherry

GEWIRTH, ALAN: THE ONTOLOGICAL BASIS OF NATURAL LAW: A Critique and an Alternative. In: The American Journal of Jurisprudence, Volume 29, Issue 1, 1984, Pages 95–121

Grisez, Germain G.: The First Principle of Practical Reason. In: Aquinas – A Collection of Critical Essays edited 1969 by Anthony Kenny

Gray, Robert: Sex and Sexual Perversion. In: The Philosophy of Sex Contemporary Readings. Fourth Edition. Edited by Alan Soble 2002

Moulton, Janice: Sexual Behavior: Another Position. In: The Philosophy of Sex Contemporary Readings. Fourth Edition. Edited by Alan Soble 2002

Nagel, Thomas - Sexual Perversion - In: The Journal of Philosophy, Vol. 66, No. 1. (Jan. 16, 1969), pp. 5-17. https://pdfs.semanticscholar.org/bc66/32ba46b9d31b0bff5285ebe8c1453513cce2.pdf

Nielsen, Kai: An Examination of the Thomistic Theory of Natural Moral Law. In: Kai Nielsen: God and the Grounding of Morality 1991 https://scholarship.law.nd.edu/cgi/viewcontent.cgi?article=1038&context=nd_naturallaw_forum

Nielsen, Kai: Myth of Natural Law. In: Kai Nielsen: God and the Grounding of Morality 1991

Nielsen, Kai: On Taking Human Nature as the Basis of Morality: An Exericise in Linguistic Analysis. In: Kai Nielsen: God and the Grounding of Morality 1991

Pickett, Brent L.: Natural Law and the Regulation of Sexuality. A Critique. In: Volume 8 Richmond Journal of Law and the Public Interest 39 (2004). https://scholarship.richmond.edu/cgi/viewcontent.cgi?article=1076&context=jolpi

Phillips, D. Z. and Mounce, H. O.: On Morality's Having a Point. In: Philosophy, Vol. 40, No. 154 (Oct. 1965), pp. 308-319

Solomon, Robert: Sexual Paradigms. In: The Philosophy of Sex Contemporary Readings. Fourth Edition. Edited by Alan Soble 2002

Weithman, Paul J.: Natural Law, Morality, and Sexual Complementarity. In: Sex, Preference, and Family - Essays on Law and Nature. Edited by David M. Estlund and Martha C. Nussbaum

Wildes, Kevin Wm., S.J. “Whose Nature? Natural Law in a Pluralistic World,” in The Death of Metaphysics; The Death of Culture (29-37). Edited 2005 by Mark J. Cherry.

Critical monographs or chapters on natural law:

Gunther Laird - The Unnecessary Science: A Critical Analysis of Natural Law Theory

Nicholas Bamforth and David A. J. Richards: PATRIARCHAL RELIGION, SEXUALITY, AND GENDER - A Critique of New Natural Law

John Corvino: What's Wrong with Homosexuality – Chapter 4: “IT’S NOT NATURAL”

Andrew Koppelman: The gay rights question in contemporary American law – Chapter 4: Why Discriminate?

Andrew Sullivan: The Conservative Soul - How We Lost It, How to Get It Back - Chapter 3: The Theoconservative Project

Shalina Stilley: Natural Law Theory and the "Is"--"Ought" Problem - A Critique of Four Solutions
https://epublications.marquette.edu/cgi/viewcontent.cgi?article=1059&context=dissertations_mu

Germain Grisez: Contraception and the New Natural Law – Chapter 2: INADEQUATE ARGUMENTS
https://catholicebooks.wordpress.com/2014/07/25/online-text-contraception-and-the-natural-law-by-germain-grisez/

John Boswell: Christianity, Social Tolerance and Homosexuality - Chapter 11: Intellectual Change - Men, Beasts, and "Nature"

T. M. MURRAY: Thinking Straight about Being Gay - Why It Matters if We’re Born That Way

Peter Vardy: The Puzzle of Ethics - Chapter 4: Aquinas, Natural Law and Proportionalism

Books on evolutionary psychology:

Douglas T. Kenrick – Sex, Murder, and the Meaning of Life

Jonathan Haidt - The righteous mind: why good people are divided by politics and religion

Joe Quirk - It's Not You, It's Biology

Matt Ridley - The Red Queen: Sex and the Evolution of Human Nature

David Buss - The Evolution of Desire

Robert Wright - The Moral Animal

Geoffrey Miller - The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature

Geoffrey Miller - Spent: Sex, Evolution, and Consumer Behavior

Books about law and the (critical) history of natural law:

Richard Tuck: Natural Right Theories. Cambridge 1979.

Stephen Buckle: Natural Law Theory and the Theory of Property. (Grotius to Hume). Oxford 1991.

Lloyd L. Weinreb. Natural Law and Justice. Harvard University Press, Cambridge, Mass. 1987.

Hans Kelsen - What is Justice?: Justice, Law and Politics in the Mirror of Science

Raymond Wacks - Philosophy of Law: a very short introduction

Books on sexuality and the history of sexuality in relation to the Church:

John T. Noonan, Jr. - Contraception: A History of Its Treatment by the Catholic Theologians and Canonists

Uta Ranke-Heinemann - Eunuchs for the Kingdom of Heaven: Women, Sexuality, and the Catholic Church

A COMPANION TO WOMEN IN THE ANCIENT WORLD. Edited by Sharon L. James and Sheila Dillon

Mark D. Jordan - The Ethics of Sex 

The Philosophy of Sex. Edited by Alan Soble

Michel Foucault - The History of Sexuality

John Boswell – Same-Sex Unions in Premodern Europe

Véronique Mottier - Sexuality: a very short introduction

Martha C. Nussbaum - From Disgust to Humanity: SEXUAL ORIENTATION AND CONSTITUTIONAL LAW

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