Gay Marriage, Incest, and Polygamy

One of the most often-mocked arguments in the debate over “gay marriage” is the argument that permitting gay marriage leads to permitting incestuous marriages, polygamy, and the like.  The  problem is, the argument is solid, and I've yet to hear a coherent answer for it.  For example, Jon Davidson evades the argument, rather than answering it:

Jon Davidson

Bringing up polygamy and incest is simply a dodge -- an attempt to distract people from the injustice of denying same-sex couples the same opportunity to marry that different-sex couples want to preserve for themselves. That others might argue that they want to marry their relatives or have multiple legal spouses requires that those arguments be separately evaluated; it doesn't make the exclusion of same-sex couples from marriage right.

So advocates of traditional marriage are right to bring up polygamy and incest. Consider two hypotheticals:

1. Harry and Larry are homosexual twin brothers, and want to marry.  Should they be permitted to?  Why or why not?
   
2. Gary, Harry and Larry are homosexual triplets, and want to marry.  Should they be permitted to?  Why or why not?

All of this is a long way around of asking, if two men can marry, why does it matter if they’re brothers? And why does it matter that there be only two of them?

Historically, the answer to both of the above hypos would be that no, they shouldn't be allowed to marry.  Marriage has been classically understood to be tied to reproduction, and regulated by traditional morality, and the notion of same-sex marriage isn't just forbidden, but literally impossible, like arguing for the legality of four-sided triangles.  Additionally, on the grounds of public morality, incest and polygamy are forbidden.

But the entire argument behind the gay marriage debate is that public morality is an insufficient basis to preserve traditional marriage, and that marriage must therefore be redefined.  But the argument that public morality is an insufficient basis to preserve traditional marriage is of momentous importance.  

After all, there's no risk of genetic deformity in the case of homosexual incest, precisely because the partners are incapable of consummating their “marriage.”  And assuming that they're twin (or triplets), the risk of coerced marriage seems minuscule - at least, no higher than in any other context. So why shouldn't incestuous and polygamous marriages be permitted, if gay marriage is?  

It would seem that the only compelling reason to forbid incest and polygamy is public morality.  Yet that's the very cultural lifeline that gay marriage advocates are seeking to destroy.  So, it's not a dodge at all for advocates of traditional marriage to raise the issue of incest and polygamy.  On the other hand, it's Davidson, and other gay marriage advocates, who are left dodging, or providing facile answers to these question.