The most common response from conservatives who raise the troubling matter of where gay marriage will lead to, is that it will lead other wacko kinds of marriages to be sanctioned -- polygamy, bestiality, incest, pedophilia, whatever.
But here's the real deal (from a comment I left here):
The next radical social-political experiment won’t have to do with marriage — that’s falling for the con that the gay marriage issue is about marriage first, and secondarily about whom it’s letting into the institution.
Clueless conservatives, or rather befuddled reactionaries, respond with, “Well, if you’re going to let ridiculous group X get married, why not ridiculous group Y? And ridiculous group Z? Where will the desecration of marriage end?”
But the culture war is not about marriage — it’s about giving fags all sorts of privileges that they don’t deserve, ignoring and indeed obscuring and denying the fact that they are fundamentally abnormal rather than normal, which justifies discrimination against them (i.e. treating them differently under the law).
That’s what makes the idea of them being married and committed such a joke, or the idea that two giddy Peter Pan homos are just as maternal and nurturing toward children as a mature woman. Or that what makes them *them* is no less healthy and wholesome than what makes heteros *hetero* — just don’t ask about how many diseases are devouring their beleaguered half-corpses.
Therefore the next big crusade in the culture war will be about “what other ways can we propagandize homosexuality as ‘just like us’ and give them goodies accordingly?” Not “what other risible group should we allow to get married?”
Look at blacks in the Civil Rights era — it didn’t move to “what other group should we allow to enter our wholesome white schools?” They didn't send in the National Guard to forcibly integrate the Mexicans, Orientals, American Indians, etc. Rather, it moved to “what other privileges, set-asides, and quotas can we shower on the blacks?”
The culture war is based around sacralizing a victim group (blacks, fags), not desecrating a particular institution (a side effect, not a sustained target).