Slippery slope?

Apparently Massachusetts is well on its way to embracing same-sex marriage as a normal part of the institution, as Kevin Drum reports here. He is optimistic that this will take the wind out of the sails of the homophobes.

The homophobes are banking everything on the proposition that same-sex marriage will lead to moral degeneracy and the breakdown of society, and when that doesn't happen they'll have nothing left.

This is an argument that I have been explicitly presented with by at least one homophobe. (His argument was the slightly more general one that societies that allow any behavior that isn't condoned by evangelical Christianity will inevitably become morally degenerate.) One hears this and imagines a slippery slope argument--if we allow gay marriage, surely soon we'll be condoning pedophilia and other obvious moral evils. Or perhaps (since the fall of Rome is sometimes cited as an example) if we allow gay marriage, it will have negative consequences for us of the economic or military kind.

However, when I myself tried to counter this argument by pointing out that plenty of nations have liberalized their laws without falling apart or turning into nations of warring, rapacious gangs, I was told that I was simply not understanding the kind of breakdown of society that my interlocutor had in mind. It took me a while to understand what he did mean, but when I did, I was shocked and mildly amused. You see, for a homophobe homosexuality is itself morally degenerate. When a homophobe tells you that allowing same-sex marriage will lead to moral degeneracy, she may mean nothing more than that allowing same-sex marriage will lead to more same-sex marriages. At least for the individual I argued with about this, the morally degenerate society in question would be one in which people were openly gay (perhaps even shamelessly talking about getting fucked in the ass!), abortions were readily available in the early months of pregnancy, the Plan B pill was available over-the-counter, etc. What the rest of us think of when we think of broken societies and moral degeneracy just isn't what they have in mind.

The argument is therefore difficult to counter. How does one argue that making same-sex marriage available to couples who desire it will not lead to an increase in same-sex marriage? (Actually, in the case of abortion this sort of strategy might work, if we view things like making Plan B available over-the-counter as part of a comprehensive effort to give women more control over their reproductive systems.) My point is that the only way to combat homophobia of this sort is to make people understand that homosexuality is not morally bad.