The arguments placed before the courts by proponents of same-sex marriage have never included plural marriage.<quoted text>
One of the assumptions that gay marriage calls into question is: why pairs? If not man-woman, then why not man-woman-woman, and so forth? The response of gay marriage proponents is generally ridicule. I don't think this is a ridiculous question. "Why can't you marry your dog, then?" is a ridiculous question.
Marriage, in our society, is between consenting adult persons. But "why only two?" isn't 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?
There are, obviously, a whole lot of societies in the world where polygamy is legal and normal. In fact the anthropological record suggests that the overwhelming majority of human societies have allowed men to have more than one wife simultaneously.
Many people who fear same-sex marriage have used that tired, old refrain--"If we allow same-sex couples to marry, where will it end?"
THEY are the ones bringing up polygamy.
The arguments in favor of same-sex marriage ARE NOT an natural leap to arguments in favor of polygamy. They have NOTHING to do with one another, other than they both discuss marriage.
Polygamists would be more likely to use heterosexual marriage rights to defend their arguments in court. This is especially true with the Defense of Marriage Act, which clearly prohibits same-sex marriage as not being recognized by the federal government.
Opposite-sex couples already have the rights and protections of marriage. Polygamists would naturally use those rights and protections that are already in existence to argue their case.