Jane Galt has kicked off an enormous discussion on gay marriage. She's the typical two-handed economist: "On the one hand, this... on the other hand, that..."
I think there's a much simpler solution:
The answer, from my perspective, is to separate the legal benefits of marriage from the cultural/religious institution.
Anti-gay marriage types say that gay marriage destroys the meaning of marriage. This may be true in some people's views, but gay couples certainly deserve equal rights with respect to taxes, next of kin issues, inheritance, etc. Civil unions accomplish this, if you give them all of the legal rights of marriage. How an anyone but the most right-wing bigot oppose that? You solve the whole problem by just not calling it "marriage."