For that matter, why does it have to be two humans? Let's just go ahead and break down all boundaries. Our paradigms about marraige are so old fashioned...
Seriously, though, I'm not sure there is a functional difference between "marriage" and "polygamy" when discussing gay men. One of the most discussed (and well documented) aspects of the gay lifestyle (again, for men, not women) is promiscuity. Women are far more demanding of fidelity in a sexual relationship than men. Remove the female quotient from a relationship, and the men tend to go hog wild. Even the gay community acknowledges this problem and struggles to come to terms with it.
So, I wonder how many gay men will "marry" simply to enjoy the legal benefits. I'm sure straight couples do this as well, but I'm equally sure the occurance of these "marriages" are somewhat lower, if not drastically lower, in the straight community. Two gay men will, as study after study have shown, have far less of a problem with their partner sleeping around than a straight couple or even a gay female couple.
I love my dog...