Well, this could then be expanded by looking at the function of homosexual relationships in nature. They are believed to have a primary function biologically speaking as well, at least in social animals that live in packs or forms of proto-societies (if they didnt, it is very unlikely that they would have survived at such a steady ratio given natural selection). If such a basic function can be discerned, then there is a natural telos of homosexual relationships as well as heterosexual ones.
This could supplement what I wrote above. Then homosexual relationships in human beings would be a higher emergent form of a basic natural function of homosexuality, as is found in several animal species, particlarily social animals.
The idea that heterosexual relationships and the generation it provides is a form of natural perfection is thus invalid, it simply is not the case in nature, since homosexual relationships seems to have its own necessary function in nature as well.
Even if you want to go the route of natural law, although I remain skeptical of its applicability (at least in very particular issues, since it seems to rely heavily upon interpretations and presuppositions), at least do it in light of current biological knowledge. You could say that there has been some developments in what we know about the biological world and complex dynamics and functions in animals and plants since St.Thomas and Aristotle.
This argument holds no water. Before I reply to it, though, I'd like to point out that you are the one, not I, who are fixated on "nature" in the sense of "what happens in nature," and it is you who are the one, and not I, who are making "naturalistic" arguments. [To be clear, my argument is not tied down to any particular doctrine of the natural sciences; the fact that you are insisting on bringing that in is just puzzling.] Just an observation.
That being said:
None of what you've said changes the fact that the animals in question are making use of the generative power of the vegetative or plant soul even when committing homosexual acts. This is a point that you seem to be ignoring deliberately: the reproductive faculty is not uniquely a power of the animal (i.e., sensate) or rational soul(s). It's a power of the vegetative soul.
You will insist that certain animals commit homosexual acts for reasons other than reproductive purposes and that homosexual acts serve a "natural" purpose ("natural," of course in the sense of biological or evolutionary or some such similar silliness; when I or an Aristotelian or a Thomist uses the word "natural," we don't mean what you mean; we mean "nature" in the sense of "phusis," i.e., in the sense of the essence or quiddity of a thing conceived as the principle of its innate drives, tendencies, etc.; the nature of a stone, i.e., its essence as a stone, is the source of its tendency to fall to the ground when dropped).
I will ask: are these animals making use of the generative power or not?
If you say "yes," then I'll answer that, by your very admission, you've admitted that your arguments are irrelevent to the case.
If you say "no," then you invite ridicule. It would be like asserting that I don't make use of the nutritive power of the plant soul when I am at a dinner party primarily for social reasons. Such an assertion is patently ridiculous and invites, not a rational rebuttal (because none is necessary), but laughter that someone would say such a silly thing.
