As I read frantic screed after frantic screed about the horrors we'll get if we allow homosexuals to marry (none of which we've seen of course), I find myself drifting to the why-not-divorce argument. Marriage, after all, was defined to be "til death do us part", or at least its been that way a long long time. Allowing divorce redefined marriage significantly, putting the lie to the argument that the definition of marriage has remained unchanged for thousands of years, even more recently than did the dominance of polygamy in the no-so-distant past. It also did so in arguably a more fundamental way than does allowing gays to marry.
Nonetheless, the real power, in this context, of considering divorce is in the threat it poses to marriage, at least from the social conservative point of view. Allowing people to marry, divorce, and remarry an unlimited number of times, reportedly caused one unknown anthropologist to describe the American social system as "group marriage, one at a time". Surely the aliens categorizing human social groups are going to see that as stranger than allowing any two people to join for life, regardless of whether their naughty bits match.
In fact, I think the right is missing out on an opportunity here. They shouldn't say "no homosexual marriage", but rather "no homosexual marriage with divorce", or better yet "no divorce at all". Of course they'd be in the small minority, especially since many of them are divorced, but at least their position would have some coherence. It has none now.