The gay-marriage movement has a bumper sticker that shows, simply, an “equals” sign. If churches are forced to participate in activities that violate the conscience of their faith, we will need to raise up that same “equals” sign in their defense. Their right to dissent is fundamental to our system.
I can't resist suggesting that maybe the reason so many oppenents get demonized as right-wing, anti-gay bigots is because they insist on making arguments against gay marriage that have nothing to do with what's actually happening across the country.
The civil marriage movement is just that: a civil marriage movement. What gay people are fighting for is equal rights under the law, not the supposed right to walk into a church, grab some hapless reverend and force him to marry two men or two women. Any religious institution has the right to refuse to conduct a religious marriage. If you want an example, just look at the Catholic church. Catholic churches won't perform marriage for a divorcee, and the fact that the divorcee has the right to be civilly married doesn't change that.
Of course plenty of reasonable people can disagree about whether a church should recognize a gay marriage. But when people argue against civil marriage by saying that then churches will be forced to perform marriage on gay couples, they either don't know what they're talking about, or are willfully distorting the truth. Which is why we tend to demonize them as right-wing anti-gay bigots.