I've got to hang with Theaetetus here. I'm all for poly marriages, but existing inheritance, tax, insurance, etc. laws don't handle more than two people well. Gay marriage doesn't run up against any existing law except the one that says "no gay marriage". You don't have to change anything else when you say, "You know all those laws about a man and woman being married? Yeah, they apply to two dudes or two chicks, too." To include polys you have to redefine how inheritance is divided, how taxes are paid, who's eligible for insurance, etc. etc.
There also need to be new laws for situations that just can't occur in a two-person marriage. For example, what happens when one person wants out of a poly group? With a two-person marriage, a divorce necessarily means the end of a marriage. With a poly group one person leaving is not necessarily the end. What about when one person is unwillingly pushed out of a poly group? Or if the group fissures into two or more sub-groups? What happens if a child of someone in the group turns 18, should they be allowed to marry into the group? Does it make a difference if the child is adopted and not the biological offspring of any existing group member? If members can be added and removed, what are the implications of a continuous marriage that can outlive all of its individual members?
The laws could be changed to make poly unions work. The laws should be changed to make poly unions work. It's just that there are a lot of things that need to be examined for that to happen. It's not just a case of going from "two people of opposite sex" to "any two people". It's a serious qualitative change that will require qualitative changes to the law. Personally I'd love to see a serious proposal to change the laws in question and allow poly marriage. I think we could borrow from laws governing corporations to put something solid together. But I'm not a lawyer. I'll let someone else do the work. I'll vote for the result, to be sure, but I'm not going to kid myself that it's just a matter of adding "or more" between the words "two" and "people".