Lawrence vs. Texas already determined that -
-what two (or more for that matter) consenting adults do in the privacy of their own home is not the concern of the government.
What is the difference between a 3 men, 1 man and 2 women, 2 women and 1 man or 3 women?
Surely you cant claim that gays have a right to privacy and polygamists don't.
The obstacle they would face is that marriage is not confined to the privacy of one's own home...