I'm not a lawyer but I never allow that fact to dissuade me from pitching in my 2 cents. I see two options here.
option 1- Marriage is legally defined as being between one man and one woman thus a clear violation of equal protection thereby nullifying all uses of marriage in legal findings and documents. Everyone, gay or straight, go get a lawyer and register your civil union at the local courthouse.
option 2- Marriage is legally defined as being between any two consenting adults and we prepare for the bigamists to come out of their hiding places and create the next big argument.
Personally I would define marriage as a religious ceremony with no bearing on law and require all couples, or any group of consenting adults, to draw up a cohabitation contract and have it registered at the courthouse. It could be for any duration or have any enforceable stipulations but the care and handling of children would need to be clearly defined.