You're missing the point.
Polygamous relationships ARE allowed. You just can't marry multiple persons and thus gain multiple perks with taxes. They could adopt each other for example.
All the gay people want is to be recognized and allowed to share the same rights as the other couples do which can marry. They want to share the name, want to be officially related to each other (and thus be able to speak for their spouse if an accident happens and the doc for example needs to make a decision).
I think its hilarious... and America calls itself the land of freedom. <3 Yes, free to do as the dictatorship pleases you to do.