Jim Ryan argues that natural law theory is a divine command theory. I think his argument misses the point that for the natural law theorist the primary issue for human beings is not biological function but rationality, and, in particular, what is involved in being a rational animal. The natural law theorist isn't appealing to divine choice at all but ultimately to truths deemed necessary. A natural law in the strict sense is just a conclusion that follows from Good is to be sought and evil is to be shunned and other necessary principles, when we add the relevant facts; a natural law in the weaker sense is a principle that has to be followed because its contradictory is conducive to violating a natural law. So for a real natural law theorist, assuming we have a natural law theorist who thinks homosexuality violates natural law, the complaint about homosexuality would not be that we have certain biological functions, but that it is irrational; and one of the reasons a natural law theorist might give for that is an argument about what role biological function should have in our practical reasoning. So the natural law theorist is not pointing to an arbitrary decision in the way a divine command theorist is; he is arguing for what would be, if he is right, a general and universal principle of practical reason. God, in fact, only enters into the matter in the sense that natural reason participates eternal reason. In other words, God's choice only enters into it in that He chose to create beings who were rational. The moral force of the dictates is derived from reason, not from divine intentions about biology, although divine intentions might be relevant to determining how serious the violation of reason is.
Natural law theory is not a specific morality; it is a theory of law that (1) understands law to be a promulgated dictate of practical reason having authority; (2) holds that there is a higher law than positive law, namely, the moral dictates of practical reason; and (3) regards the principles of the higher law as being universal, for the same reason that any principle of reason is universal: anyone who violates the principle violates reason. This is why any charge of 'naturalistic fallacy' against the natural law theorist is absurd in the first place; the natural law theorist doesn't consider nature as such, but reason. In the face of the charge, the natural law theorist doesn't have to retreat to divine intentions, but simply to point out that the person making the charge is effectively making the absurd demand that morality have nothing to do with practical reason.