Problem with this form of argumentation is that it restricts the purpose of a natural function.
This is presupposed in the very notion of a faculty. A power or a faculty is related to its activity or operation as first to second act (being vs. doing). The operation or activity of a faculty is the due perfection of that faculty.
It is a form of what I can only call telic reductionism.
There's already a word for that. You could have replied that it's a "teleological moral argument." And I'll answer you that teleology is presupposed in the case of every power or faculty.
Morality ultimately is about acting according to right reason. The first principle of practical reason, as St. Thomas tells us, is "do good and avoid evil." [Here, against Kant, there is no need to distinguish between "moral" good and evil and the "good" and "bad" which appertain to animal satisfaction and are more properly called "doing well or poorly." There is a prior, ontological sense of "good" which is convertible with truth and being. Reason, having appertained the truth or being of the thing in question, subsequently recognizes, in its practical considerations, the good to be done.] There is an intrinsic deformity, which reason recognizes, in using a power or faculty with the expressed intent of frustrating or circumventing the end or goal of that faculty.
It is a form of argumentation that also relies on empirical observation of nature and reasoning based on that. It is evident in thise case that it is rather selective in what it observes. Animals do not merely use sexuality for reproduction, observe an ape and you will see that they more than just reproducing. Of course, you can argue, and rightfully so, that animals and nature is not a good guide to right and wrong, but then you undermine your own natural law argument as well.
Irrelevent.
Further, to reduce the purpose of eating to mere nourishment is just as inadequate and ignores the cultural significance of the practice. The raison d'être of meals might be nourishment, but to reduce it to that is simply overzealous reductionism. More complex purposes of eating has emerged from it, a feast is more than mere nourishment of more than one person, it is a cultural event with a multitude of social functions as well (and nowhere is that clearer than in the feast culture that make up the culture of antiquity). Similarily, the purpose of sexuality cannot be reduced to mere reproduction, it is the raison d'être biologically speaking, but more complex functions has emerged from it in our cultural and biological history. It is the ultimate expression of affection, intimacy and love (love here used in a multifaceted way, it cannot be reduced to merely eros) between two people. If that is granted, the case for ruling out homosexuality based on the biological raison d'être of sexuality becomes questionable to say the least.
Once again, all of this is irrelevent. Reread the argument that I've given. At no point did I say anything like: "The sole 'point' of any given act of sexual intercourse, in every animal instance of it, is reproduction." Yes, it is true, as we ascend to higher forms of life (brute animals and human beings), the lower powers can take on new kinds of significance (though these powers do not take on these new kinds of significance
insofar as they are those powers). I fully grant that. In human beings, sexual intercourse becomes a kind of political or social activity (i.e., in the "society" of the household of a man and his wife),
insofar as the human being is a rational and political animal.
But ultimately, this point has absolutely no bearing on the point that I made above. The generative power
as such is not related to the social union of a male and female animal as first act to second act. Its due perfection
as such is generation, and this is true regardless of what kind of living thing we are talking about.