Well, it looks like California isn't alone. Shortly after gay marriage was legalized in Maine, a ballot initiative de-legalized it, similar to what happened in California with Prop. 8. Here though, the Maine campaign was, by all accounts, fantastic (unlike the California No on 8 campaign), but the turnout wasn't high enough.
The one common thread in both campaigns, though, is messaging. In both instances, the message was "don't take away our rights." The anti-gay marriage crowd responded by proclaiming that if gay marriage continues, then gay marriage will be taught in schools. And in both cases, the response by the gay rights groups was to dispute the notion - after all, the whole gay-marriage-will-be-taught-in-schools thing is nonsense.
And this is where I think the messaging is wrong. Look, gay marriage is a BIG change for our culture (and even bigger if you live in Maine). Homosexuality has only been legalized throughout the country in the past ten years. So, rather than downplay the importance, play it up. Humanize the issue by having old gay and/or lesbian couples talk about their relationship in ads. Have survivors tell their stories about losing their partner and the aftermath. In other words, run ads in FAVOR of gay marriage, instead of OPPOSING taking gay marriage away.
With regard to the schools thing, don't deny that gay marriage will be taught in schools - because its a BIG change, some teacher is certain to bring it up - instead, point out that the whole "its going to be taught in schools" thing is scary only if you don't want to accept gay people. Tell them that even though its a BIG change, in ten years, it will seem like a trifle because nobody really cares about who marries who except those getting hitched.