"In the eyes of the government and courts of California, marriage shall be hereby defined as none of its goddamned business."If gay people are ever allowed to get married in California, it won't be by convincing straight people to care. That isn't to say that straight people don't care; 2,909,370 people voted against Proposition 22. Gay people will win the right to marry when straight people start getting uncomfortable about having the government rummaging around in their married life. Then gay people and straight people can rally behind the same slogan: It's none of your goddamned business. A proudly American sentiment.
Democrats reflexively package their arguments in the terms of wonderful programs and insightful social policy. This isn't going to work for these kinds of issues. Most straight people still feel uncomfortable about actively supporting a policy that will lead to men kissing other men on TV. But most straight people probably would support a policy that would grant them a little more freedom and privacy.
From there, it's easy. Americans will generally be comfortable reasoning that what other people do with their privacy and freedom is their own damn business. It's simply a matter of framing the debate in such a way that the bible thumpers are bossing "regular people" around.