http://en.wikipedia.org/wiki/Marriage#History

For most of European history, marriage was more or less a business agreement between two families who arranged the marriages of their children. Romantic love, and even simple affection, were not considered essential. Historically, the perceived necessity of marriage has been stressed.

In Ancient Greece, no specific civil ceremony was required for the creation of a marriage - only mutual agreement and the fact that the couple must regard each other as husband and wife accordingly.
Like with the Greeks, Roman marriage and divorce required no specific government or religious approval. Both marriage and divorce could happen by simple mutual agreement.
rom the early Christian era (30 to 325 CE), marriage was thought of as primarily a private matter, with no religious or other ceremony being required. Until 1545, Christian marriages in Europe were by mutual consent, declaration of intention to marry and upon the subsequent physical union of the parties.
Marriage out-dates Jesus. This is why the religion aspect is a flawed argument. It was only decided that marriage was only official if done by a priest during the counter-reformation when the catholics got all pissy about being marginalized and persecuted etc. Surely we've moved on since then?

I'm not arguing that churches should be forced to hold gay weddings, I feel that should be up to the church. The point is that marriage is not - and never really has been - a specifically religious idea. Therefore the christian lobbyists should stay out of government business when it comes to granting marriages to couples who love each other.