I suppose it is possible that one could argue a 14th amendment extension of the 1st amendment to the state level, making it more than just an issue for state's to resolve.
I would vote for my state to completely get out of the marriage business. I understand the need for state-recognized unions for the purposes of tax, right to property, etc. I would be fine with my state issuing union licenses to those who have been married by a state-recognized church or to those who come to the state/city for a union. To me, marriage is rooted in religious foundations - so I don't think that state should have the right to issue a marriage license. If you want to get married, go to a church. If you want to be recognized by the state/federal government as being in a union, take that marriage certificate to the state/local authorities and have it recognized.
States could still vote, I suppose, to not allow gay union (or any other kind of union, I guess), but I just think it is a business the states shouldn't be in as long as we define marriage as a sacrament (which I think most of America does, though perhaps that's another vote). Though even as I say that I worry about saying "that's another vote"....everything can't be put to a vote...we've already been down that road with regard to evolution, IIRC.