A very interesting read in The Economist about the link between same-sex marriage and opening the door to polygamous marriage.

“One of the assumptions that gay marriage calls into question, for many conservatives, is: why pairs, then? If not man-woman, then why not man-woman-woman, and so forth? Again, the response of gay-marriage proponents is generally ridicule. I don't think this is a ridiculous question...It's easy enough to show that gay marriage does not empirically lead to pressure to legalise polygamy; that hasn't happened anywhere that gay marriage is legal. But this is different from explaining why opening up the boundaries of the 20th-century understanding of marriage shouldn't raise the possibility of legalising polygamy. Why shouldn't it be legal for more than two consenting adults to marry each other?”
It is interesting that this problem with same-sex marriage has been getting an increasing amount of attention. As Carson Holloway at The Public Discourse pointed out, Justice Sonia Sotamoyor also raised this issue during the oral arguments on Proposition 8.
Note that this argument is not: if same-sex marriage is allowed on Monday then polygamous marriage will be allowed on Tuesday. Rather, the argument is that defining marriage as a union between any two adults is logically inconsistent. If the gender of the partners in the relationship is dismissed as arbitrary and irrelevant to marriage, then there is no basis for why the number of partners is relevant either. If marital history and tradition is put aside in the legal definition of marriage, then the traditional western concept of marriage being between only two adults has no standing.