Actually, this has nothing to do with religion and has to do with all people being treated equally in the eyes of the law. Like it or not, the government, and its citizens, have a real interest in governing marriage - not in churches, but as it relates to society as a whole. Having codified laws related to these specialized contracts (which cover property distributions, inheritance, divorce, child custody, etc.) are necessary to keep the court system (and by extension our society) moving.
The actual problem comes with the fact that since the beginning, non-government, non-elected agencies/organizations (organized religion) have been allowed to determine who is eligible to the right to enter into this completely governmental marriage contract. You can't have that. Again, just because a gay couple is allowed to get a marriage license from the state to enter into a marriage contract (and that's all it is as it relates to the state, a contract that is governed by a specific set of laws) how does that take away from anyone else's religious marriage ceremony? It simply doesn't. No one is forcing churches to perform ceremonies for gay couples - even though some churches are doing it voluntarily - and the "definition" in the churches will never have to change.
And really, if including gay couples in the state's "definition" of marriage takes away from someone's religion, then I severely question their conviction and faith. What the state thinks should have no bearing on a person's religion if they have real faith - a state's laws have no bearing on God's law.
