They will be on much firmer ground than the gay marriage people, being able to point that polygamous marriage is recognized in many (Muslim) countries around the world, and for the US to refuse to recognize polygamous marriages legally performed overseas is discriminatory.
After that will be lawsuits striking down age limits on marriage. A marriage between a 40 year old man and a 10 year old girl, legally performed in Saudi, must be recognized in the US, otherwise it is discriminatory.
The slippery slope is now greased.
Especially when there are Mormons and some radical liberals arguing for it, like the lesbian “throuple” in the UK, homosexual male trio in Thailand, and a California trio of two lesbians plus the guy they added, with an article gushing that each baby has both a Mommy and a Daddy plus another Mommy!
Watch out for this statement from Kennedy's majority opinion - this opens the door for all kinds of crazed "liberties" created by the Vicars of Obama. Just imagine your worst nightmares