- Utilitarianism? Well, on the one hand it is pretty plausible to think that only human wellbeing matters. But on the other hand, "it would justify sodomy!"
- Kantianism? It's all well and good to treat other people always as an end in themselves, and never as a means only. Then again, once the universalizability constraint is properly understood, that too "would justify sodomy!"
- Contractarianism? It tends libertarian, allowing people to do what they will so long as they don't harm others. But, gosh, you know what? That too would justify sodomy! Oh no!
- How about ethical egoism? Surely that can be relied upon to give ridiculous results. Alas, not in this case. Given that gay individuals are going to be better off having loving sexual relationships than being lonely and miserable their whole lives, even this poor excuse for a moral theory is going to justify sodomy.
And so forth. Try a few others for yourself -- you'll be amazed at how easy it is to refute otherwise plausible theories using this one simple objection. Who ever said there's no progress in philosophy, eh? In a few short minutes we've managed to overturn centuries of moral philosophy! All that's left are those few moral theories that are completely arbitrary.
Divine command theory might work -- God can command whatever he likes, so he could stipulate that sodomy is wrong, and rock music too. That's gotta count for something. Otherwise we might try moral relativism: if conservatives think that sodomy is wrong, then it really is "wrong for them", no rational reasons required. Or we could ask the magic 8-ball -- at least there's a chance it'll answer "no" when we ask it whether sodomy is justified, and we're really running out of options since we ruled out all the non-arbitrary moral theories already. How about flipping a coin to decide?
I guess we might resort to natural law theory, and claim that sodomy somehow violates the natural "purpose" or "function" of the sex organs. But good luck explaining why clapping isn't immoral, then, given that hands are for grasping. The aforementioned Right Reason post tries to draw a distinction between using an organ for "other than" its purpose vs. "contrary" to its purpose, where only the latter is "immoral". But then - though the author refuses to recognize it - that would justify sodomy, because sodomy only temporarily disables the sex organs, just like clapping temporarily disables my hands, but either can be put back to their "proper" use soon afterwards. I can see no principled difference between these two "unnatural" practices, insofar as their relation to natural purposes is concerned. (More here.)
Devastating though the "but it would justify sodomy!" objection no doubt is, there's more to be said against natural law theory. For one thing, it's patently false. I've talked before about "Creator's rights" and the illegitimacy of externally-imposed purposes. Quick refutation: suppose humans had a scorpion-like tail, the (evolved or God-given) purpose of which was to kill our enemies. Further suppose that modern scientists could remove the tail and extract useful medical substances from it. The idiot's law theory implies that it would be wrong to use the tail to save lives in such a way, for that would be contrary to its "true purpose" of killing!
Clearly, such externally imposed "purposes" have no intrinsic bearing on morality. They're morally arbitrary. What little plausibility this theory has arises because evolution has tended to provide us with organs whose natural functions promote our wellbeing. And of course it's our wellbeing that really matters. But a few conservative dullards get confused by the correlation and decide that it's the natural functions that really matter. How depressing.
Anyway, I was supposed to be ranting about morally retarded homophobes, not natural law theorists (though I suppose the two categories do tend to overlap -- at least when the homophobes can think of no other rationalization besides "it's unnatural!").
I must say, one good thing to come from all of this is the sheer entertainment value. We've already had wingnuts arguing for their (purportedly) factual claims by asking: "If you doubt this is possible, how is it there are PYGMIES + DWARFS??" Now we can complement this with the similarly inane moral objection: "But that would justify sodomy!" As with the "pygmies + dwarfs" remark, it can be applied to just about anything. That's sure to come in handy.