Gay civil marriages ruin the sanctity of religious marriage to the same extent that gays eating crackers ruins the sanctity of communion. That is to say, inside the religious congregation it has no effect, and outside the religious congregation it has absolutely no effect.
While California is temporarily shading its eyes from the Enlightenment that has proven no trouble for the likes of Massachusetts, Connecticut and Iowa, I believe it won't be long before the California State constitution gets reamended* and every person previously denied the privilege will be able to wed the person they love.
There is a certain tack I'd like to see the state take in doing this. One of the biggest talking points of the Prop 8 scum** was that their churches would be obligated to recognize and marry gay couples if gay marriage were legalized. Nevermind the fact that this isn't remotely close to the truth, let's make sure that when we finally and permanently legalize gay marriage in California, we explicitly state the difference between civil marriage and religious marriage. Let's emphasize that churches will retain the right to refuse service to anyone not wearing a shirt, shoes, or who prefers genitalia that match their own.
Make them wear their bigotry on their sleeves. The more publicly their distasteful behaviors are presented, the sooner society will choke them off.
Let's hope that brave gay couples who want a religious wedding can stand up for themselves and leave congregations that won't support them, and move to more progressive and forward-thinking churches, or better yet, away from churches in general. Politically active religious groups feed on people, and need to be starved into submission and out of relevance.
* - unamended? remended? remedied? repealed? remodeled? role-modeled? Something like that.
** - a kinder word than I really wanted to type.