Haller addresses three varieties of arguments against same-sex relations, and appears to regard all of them as variations on the theme of Natural Law arguments. Of the three he describes, only the second one even comes close to being anything like a genuine Natural Law argument, but even that one falls far short of what a proper argument from the Natural Law would amount to. Now, of course, we're all just writing blogs here, so one must adopt Aristotle's adage from the Nicomachean Ethics and remember that "the same exactness must not be expected in all departments of philosophy alike" (I.3 1094b12). On the other hand, if one is going to be shooting fish in a barrel one had better make sure they are not red herrings. It simply will not do to trump up three ridiculous arguments against same-sex relations, label them "Natural Law" arguments, and then proudly proclaim that one has shown "Natural Law" arguments against same-sex relations to fail.
I will pass over in silence the first and third "arguments" that Haller addresses, as they are evidently straw men. But as I remarked above, the second argument does bear some slight resemblance to an actual argument from the Natural Law, so I want to say a few things about it. Here's the "argument", along with Haller's proposed reply:
Now, of course, my interlocutor will then advance to the second form of natural law and say, “By ‘unnatural’ I mean ‘not in concert with the ends for which sex is intended.’” This natural law position falls prey to the fallacy of begging the question, in that it rests on two prior premises, neither of which is self-evidently true: “Moral value is to be found in the use of things in advancement of the end for which they are intended” and further, “Sex is intended for procreation.” I have already demonstrated the falsehood of the latter premise, in that while sex may often result in procreation, there is ample evidence that it serves other purposes or ends having nothing to do with procreation. This is, in fact, the position of The Episcopal Church, as enunciated in the preface to the matrimonial rite, where procreation ranks third among the ends of marriage, “when it is God’s will.” It is also the reason marriage, and sexual activity within marriage, are not forbidden to infertile couples, and birth control is permitted.Although there are some appeals to Natural Law that are similar to this one in certain details, no genuine such argument is precisely like this one. The principle difficulty is, perhaps, due to the informal nature of blogging, because the argument is summed up as "By 'unnatural' I mean 'not in concert with the ends for which sex is intended," and no natural law theorist would put it quite like that. First, the expression "in concert with" is hopelessly vague; second, the notion of "end" being appealed to here is undefined; and finally, "intended" suggests a principle of design that is simply absent in genuine natural law accounts. I think, however, for the purposes of analysis, we may assume that what Haller has in mind is some sort of appeal to the necessity of final causation, where precisely defined ends are related to functionality in one way or another.
If we make such an assumption, we are left with plenty to say about Haller's proposed reply. First, there is his claim that such an argument presupposes that "moral value is to be found in the use of things in advancement of the end for which they are intended". I would be willing to bet money that he thinks this is presupposed because of the use of the word "unnatural", and yet to use such a word implies nothing at all about "moral value". To say that something is "unnatural" is simply to say that, in general, things tend to work differently in natural settings. We all know that water tends to freeze at 32 degrees Fahrenheit when the pressure is one atmosphere, and we also know that altering the pressure or the composition of the water can alter the temperature at which it freezes. If we know that the water's composition has not been altered, and that the pressure is one atmosphere, it would, in this sense, be "unnatural" if water were not to freeze at the specified temperature. But that is not to say that it is logically impossible for it to do so, or that it would be anything like a moral outrage if it should occur.
Then there is the claim that another suppressed premise in the argument is "Sex is intended for procreation". Here Haller appeals to that notion of "intention" that he left so conveniently undefined earlier. In the present context it is entirely unclear what it means to say that sex is "intended" for anything. What sort of "intention" are we appealing to here? Is it the "intention" of some intelligence, that is, some agent acting with purpose? If not that, then what? Mere adaptedness, the byproduct, perhaps, of natural selection? It simply isn't clear in what sense we are to take it or how, taking it one way rather than another, the desired normative implicature would result. The sense, however, that is usually behind a Natural Law argument is that of final causation. That is, rather than saying that "sex is intended for procreation", the natural law theorist will say that procreation is the final cause of the sexual act. This does not entail that procreation is "intended" as a consequence of every sexual act, but rather serves to explain the function, or role, of the sexual act in a larger ecology.
This sort of claim may not satisfy everyone, of course. It is notoriously difficult to specify these sorts of final causes without appealing to evolutionary just-so stories or else material tautologies. By a "material tautology" I mean something like the following. Consider the question: What is the heart "for"? Suppose someone were to posit "The heart is 'for' the circulation of the blood" and someone else were to object "That's arbitrary--the heart does other things, too: it makes a rhythmic sound, it helps with oxygenation, it contributes to the overall body temperature." The objection seems strange: all of the posited alternatives--making a sound, oxygenation, temperature regulation--are materially identical to the function of circulating the blood, that is, they are present simply as an artifact of the blood being circulated by rhythmic contractions of the heart muscle. It seems clear that circulation is really the only thing that the heart is really "for", if it is "for" anything at all. But this sort of analysis will not do in other kinds of cases. What is the human mouth "for"? Is it for articulation? Mastication? Defensive biting? Sexual stimulation of another? These alternatives are not materially identical to each other, they are quite distinct, and they are all things that people have, from time to time, used their mouths for. So here one might appeal to evolutionary biology and try to pinpoint the selection pressures that gave rise to the mouth as it is presently structured. But this sort of exercise is plainly speculative and can bear no normative weight, since such hypotheses are always open to revision. The principle difficulty in a case like this is the fact that the mouth, quite simply, has many uses, and none of them stands out as definitive, rather, we might just as well say that what the mouth is "for" is the whole suite of them.
Is this the case with sexual activity and the sex organs? What is the sexual act "for"? Procreation? Mere pleasure? The unification of individuals? To communicate love? Haller, like many others, wants to claim that sex is really for all of these things, and not at all for any single one of them in a normative sense. In a strange irony, Haller alludes to a science that is morally neutral in defense of his thesis that one cannot infer an "ought" from an "is", and yet if we accept the truth of this--as we should--then science itself offers a rather straightforward refutation of his view that sex "serves other purposes or ends having nothing to do with procreation". The "other purposes" that he has in mind are, apparently, things such as were just described, and yet all of these things seem rather obviously materially identical to the act of procreation. From an evolutionary point of view, that is, the fact that people who have sex with each other sometimes also fall in love with each other means only that there is differential reproductive success attached to the emotion of love. Love is nothing more than an instrument of reproduction on an evolutionary account. Haller appeals to the fact that sex sometimes fails to produce offspring as evidence that procreation is not what it is for, but that is tantamount to saying that the fact that some batters strike out is proof that going up to bat is not for hitting the ball, so I think we may pass on that one. The point is that, biologically speaking, everything involved in sexual activity is directed towards reproduction, especially if the theory of evolution is true. On this sort of a reductive, materialist scientific account, same-sex relations are the ones that serve no purpose--indeed, they are virtually deleterious and are, hence, selected against.
Haller is right about one thing: the expression contra naturam does not, in and of itself, entail "morally vicious." To move from the claim that same-sex relations are contra naturam to the inference that they are thereby morally wrong does require some set of reasons for thinking that it is better for ends to be achieved than not. I won't bother to give that argument here, since Aristotle does it himself much more thoroughly than I ever could in his Nicomachean Ethics. The key lies in seeing sexual relations not in the isolated way in which Haller foists upon his alleged "natural law" theorist, but in the integrative, holistic way in which the virtue ethicist understands the final cause of the human being. This is the way in which the Church has traditionally deployed arguments from the natural law, and it is rather surprising that Haller would not even bother to address this form of the argument, given that he describes himself as a member of a (Christian) religious order. One begins almost to suspect him of special pleading.