That's the problem with the "natural law" approach: if it really is a law of nature, then that's how it will be in any case, and there's no need to enforce it. If it's not always the way it is, then it's not actually a law of nature, is it? "Natural law" always boils down to people taking one view of things that they prefer, for whatever reason, and making a kind of appeal to authority for their view, based on the claim that it's the mandate of "nature."
Me, I think "natural law" is always a smokescreen for divine command theory. At least, that's always what it's been in my experience.
And yes, the fact is that homosexuality exists naturally in much of the animal kingdom, including in some of our closest genetic relatives. If one must believe in a creator god, then one must either concede that the creator must like homosexuality, or else he must have made a lot of mistakes along the way. Or one could just concede that the taboo was created by humans for very human reasons and then put into the mouths of gods in order to give it authority, as they have done so many times before, with so many terrible things.