If the SCOTUS decides that gay marriage is a Constitutional right, polygamy will soon be legalized and laws against incest will also be struck down.
If sodomy isn’t illegal and if sodomites can marry, there’s no logical reason to prohibit polygamy, bestiality, incestuous relationships and marriages and so on. Make no mistake, the left is already on a crusade to lower the age of consent. The rest of this is on their radar as well.