John Holbo knows what's up:

What do the [National Review] editors, and Gallagher, really think? The ick argument, I’ll wager. They want to stop same-sex marriage as a way of sending a message of ‘ick’ to gays, and about gays. But they also don’t want to be labeled homophobes. That is, although saying ‘gay marriage shouldn’t be allowed because I believe gay sex is icky’ is actually a less terrible argument than anything they’ve got – hey, it’s not flagrantly internally incoherent, it’s basically honest (I’ll wager), and who doesn’t believe that on some level people steer, morally, by emotional attraction-repulsion drive? – it’s considered embarrassing. (Homophobia: the yuck that dare not speak its name.) And, even if it weren’t embarrassing, it’s obviously not strong enough in the current environment. So what do you do? You end up thoughtlessly backing into something that’s frankly orders of magnitude worse than just saying gay sex is icky. Namely, gays are un-persons, so far as the state is concerned.

What makes these arguments so weird is the mildness of the underlying opposition to homosexuals and homosexuality – the implicit inclination to be basically tolerant. ‘C’mon, gays, you know you’re ok, and we know you’re ok, and you even know that we know you’re ok, but we don’t like it, so can’t there be some way that we can insist on us being a little better than you? It can be a small thing. Symbolic, but slightly inconvenient for you, so people know it’s also serious?’