This is not directly about gay marriages although the issue got me thinking about it. I would believe the same way even if there were no gay people that were fighting for marriage rights. It is also not about bringing all people down to the lowest common denominator.
As I see it, marriage is composed of two parts. One is the religious and/or symbolic piece and the other is the legal part granting certain rights by the state. Even today, the religious component can be performed by a Catholic Priest or a friend that got his credentials through mail-order. The legal component is given by getting a “marriage” license through the state and in some states, signing papers after the ceremony is completed.
I propose that states should only be in the business of civil unions. That is, a legal contract between two people to have legal rights that union affords them. However they want to do the marriage/symbolic part is up to them.
Realize that this is strikingly similar to what happens today in practice but the philosophy is very different. States should not be in the business of granting marriages in the religious/symbolic sense because of the separation of church and state but should recognize it as a legal matter parallel but not synonymous with that joining of people. They should only grant it as legal privileges and these should be called civil unions according to the law.