The concept of marriage predates the governments role in it. It is a religious concept, like Pudz said.
I'm for gay marriage, in the legal sense. Religious institutions though should be able to maintain their practices. Which is generally the case in places where gay marriage is legal.
But for some reason, just the term "marriage" is difficult to swallow for a lot of people. Even if the religious institutions aren't affected by it. Gays should not have their own term - like "civil unions". It should be equal.
So, as Pudz suggested, if we strip the concept of marriage from the government and leave it to the religious institutions and come up with a new legal concept, like civil unions but as Shining Raichu said, with a nicer more romantic name) that applies to everyone gay and straight that might make it a bit more palatable for some.
When a straight couple gets married in the church they would then also receive a "nicer name for civil union" certificate which a gay couple would also receive when getting "nicer name for civil union-ed" at city hall. Equals rights, with marriage returning to a religious role with the legal aspect being something different (yet simultaneous). You can only get married then if it is part of your belief system, just like baptism for example... unless, of course, Mitt Romney does it to you after you're dead.