I think that both sides - left and right - in this debate rest on faulty arguments and reasoning because the fact that something occurs in nature does not thereby make it ethically right for humans, and the fact that something does not occur in nature does not necessarily make it wrong for humans.
But no informed person, natural law theorist or otherwise, says it does. It is not 'occurring in nature' that is the standard; it is 'conformity with our nature as rational animals' that is the standard, for the very simple and straightforward reason that it must be taken as a fundamental part of our practical reason. Thus both sorts of arguments Eby notes, that of Catholics and that of homosexual rights activists, are entirely legitimate arguments. Of course, one can debate whether either is getting the basic principles of practical reason quite right. But the homosexual rights activist (for instance) is not saying that it is the mere fact of biology that decides the issue; it is the role he or she thinks that fact must play in our practical reasoning that is the real heart of the argument. And Eby's argument doesn't do anything against this line of thought. The same must be said of the Catholic arguments, which Eby's argument doesn't touch, either. Eby concludes:
Some might want to claim that humans, as thinking and ethical beings, can and should make judgments about which of nature's activities - which of nature's "is" statements, so to speak - we should accept as norms, or "oughts," and which we should not accept. I agree fully with that claim. But note that this claim cannot be itself derived from nature. It is extra-natural and a priori to our investigation of nature. This means that our human ethics does not and cannot rest just on nature, but must be from some source "above" or other than nature.
But this can only be the case if we cordon off human beings themselves from nature. If we are seriously interested in understanding the nature of things, however, we must understand our own nature. And it is by no means obvious that when we begin considering our own natures that we find the is/ought divide to be so clear as Eby thinks. [Whoops, tangled myself up in that sentence and ended up saying the opposite of what I wanted to say. Fixed it.--ed.] For a human being is an 'is' that clearly has some relation to 'ought'. In other words, if we take human nature seriously, it's not so clear that ethical norms "must be from some source 'above' or other than nature". It's only by gerrymandering nature to exclude our own nature that we get this claim, or so it seems.