For a long time I've actually thought that once gay marriage was legal, laws against polygamy would probably declared unconstitutional as well, based on similar legal reasoning.
Even Chief Justice John Roberts basically said as much in his dissenting opinion to Friday's Supreme Court decision on same-sex marriage:
It is striking how much of the majority’s reasoning would apply with equal force to the claim of a fundamental right to plural marriage. If “[t]here is dignity in the bond between two men or two women who seek to marry and in their autonomy to make such profound choices,” why would there be any less dignity in the bond between three people who, in exercising their autonomy, seek to make the profound choice to marry? If a same-sex couple has the constitutional right to marry because their children would otherwise “suffer the stigma of knowing their families are somehow lesser,” why wouldn’t the same reasoning apply to a family of three or more persons raising children? If not having the opportunity to marry “serves to disrespect and subordinate” gay and lesbian couples, why wouldn’t the same “imposition of this disability,” serve to disrespect and subordinate people who find fulfillment in polyamorous relationships?
Here are a range of other voices weighing in on the relationship between gay marriage and polygamy.
Some more recent:
Politico: It’s Time to Legalize Polygamy
Informed Comment: No, GOP, biblical Marriage was not between one man and one woman
And some from a while back:
