Since the first settlements at Yorktown these pillars of marriage have been sacrosanct. In the 19th century the federal government sent troops to Utah to suppress polygamy, for example.
Gays want to dump part two of the tri-def of marriage. Sorta pick'n and choos'n as they say. But if gays somehow manage to get what they want and obliterate the opposite sex part, then there really is nothing to stop the train from roll'n further down the track.
Yesterday Slate Magazine published an article, "Legalize Polygamy, No I'm Not Kidding." The cynical author pretends to answer critics of gay marriage. Author Jillian Kaneen gets cute in her little tome but her sarcasm is lost on the seculars that agree with her.
Marriage and government don't mix and that's where all the trouble's brewing. Government should only sanction the civil portion of said unions. Leave social and religious institutions the privilege of codifying the rest.
Of course the first time a man tries to marry his pit bull, or a woman tries to marry her Shetland pony the cops will show up. So, at least for now, there will still be 'limits' on the marital messes morphing even further into the realm of the bizarro world.