Who Invited You?

Like a majority of Americans in my age group, I'm in favor of legalizing gay marriage. However, my law school-enrolled wife noted that it'd be interesting to see how public support would fare for the groups certain to follow gays into this public dialogue: polygamists and people in incestuous relationships.

It seems that her test case maybe be on its way, as a convicted Utah polygamist claims that the Supreme Court's decriminalization of homosexuality means that polygamy should be made legal. His case would probably be stronger if he weren't facing life in prison for having sex with one of his wives when she was 13.

Although Tom Green's adolescent wives will probably keep him off the streets, more palatable non-monogamists are sure to emerge. Pundit-worthy incestuous couples will follow.

...and these ways of life will gradually all become legal. As well they should.

From where I sit, Leviticus 18:22, Thou shall not lie with mankind as with womankind: it is abomination, warrants as little attention today as Leviticus 19:19, Do not wear clothing woven of two kinds of material. I doubt that sexual morality laws will end up hinging on much more than involving consenting adults and avoiding cruelty to animals.

The only real question is timeline.

Suppose gay marriage is legal in the majority of American states in 2020...
When will polygamous marriage be legal?
2050? 2075? 2100? 2500?

How long until incestuous couples can enter into legal unions?
Since inbreeding is famously unpopular, and children raised communally generally exhibit the Westermarck effect (whereby they're not attracted to one another), I'm sure incest will take longer before it becomes legalized.

...but how much longer? 10 years? 100? 500?