I ask this from the perspective of the government. Why does it have any role in marriage? Why does it care the bonds and contracts people make, as long as they are not exploitative or illegal?
I've thought that marriage should be entirely ignored by government and left as a religious or cultural institution, to be decided however they feel like, and let government instead have a completely independent system of civil unions. If a church or society wants to define marriage one way or another, let it, the government should have no part in it at all.
But that still leaves the two-person bond recognized by government. Why should there be this special contract and legal arrangement between two people and only those two people?
That's a question I've never seen come up in the debate over gay marriage, but it seems critical. If we don't define the purpose of marriage/civil unions, how can we decide who can marry and how? It would be like trying to design and pave a road, without deciding where it should go.