I don't see a problem: marriage is a religious institution, a sacrament in some churches, and the union itself is a civil matter (not a 'civil union' as it's used in the press). If a church forbids same-sex marriage, denying the blessings of that religion to those entering into a union, that's the church's business. If a state wants to allow unions between members of the same sex, that's a civil matter. We don't have to confuse the two, but we have.

Or, perhaps better yet, call the religious rites "matrimony" and the civil rites "marriage".

Now let's get back to the "render to Caesar/render to God" thing. Get hitched in a church, before God(ess), you're engaging in matrimony. Get hitched before a JP, judge, or the mayor, you're married.