TNR's got a good summary by Jonathan Chait of how the arguments against same-sex marriage are all pretty much ridiculous bullshit. Here's my favorite part:
The most striking thing about anti-gay-marriage arguments is that they dwell exclusively on how heterosexuals would be affected. Heather Mac Donald of the conservative Manhattan Institute writes, "I fear that it will be harder than usual to persuade black men of the obligation to marry the mother of their children if the inevitable media saturation coverage associates marriage with homosexuals."But read the whole thing, there's plenty more to enjoy.I suppose you could imagine, somewhere, a black man telling his friends he's going to propose to his pregnant girlfriend, only to be taunted, "Marriage? That's so gay," and think better of it. I don't find this very likely.

It's a nice logical argument. There are plenty of simple, infallible logical arguments to make about the subject.
The proponents of the ban have no interest whatsoever in logic. They don't want or need to explicate their "reasons" because reason has nothing to do with it. It's a fear-based, religiously grounded agenda. Rational thought has no business sticking its dick in that.
Hilarity ensues when they conflate it all with American Values and pop the anal cherry of the First Amendment.
I think what they really mean is, how can they possibly force closeted men to marry their conservative evangelical harridan love-manques when they might be able to ride off to a Bear-Fest up in Vermont and mudwrestle with their gay husbands (and the other gay bear husbands, of course).
This is all pretend reason. Actual reason will get you kicked out of the churches and backwaters where all this pretend reason comes from.
Wait, no. Actual reason will simply make you stand up and walk out.
So they are worried the gay marriages are going to cut into the sham marriage market?