Perhaps we need to clarify what calling something a marriage is. To even have marriage, gay or otherwise, you have to say that this is something they are doing over and above hooking up and shacking up.
Different groups within society can issue the status of marriage, and they will do so based on their own religious convictions. The Southern Baptist Church will use different criteria from the Metropolitan Community Church.
The fact that we have marriage licenses suggests that the government has an interest in identifying and sponsoring relationships that have this status. But what, exactly, is that interest? And should there be such an interest? That is, I think, the proper focus of the gay marriage debate.