As Ben Smith notes, all three candidates have said the issue should be left up to the states, but only John McCain actively supported a gay marriage ban in his own state, Arizona. That initiative failed at the polls in 2006, making Arizona the first state in the nation to reject such a measure once it reached the ballot.
In any case, I was fascinated yesterday by the fact that the Obama campaign quickly sent out a press release in response to the California ruling, but the Clinton campaign did not. Here's what the Obama folks had to say:
Barack Obama has always believed that same-sex couples should enjoy equal rights under the law, and he will continue to fight for civil unions as President. He respects the decision of the California Supreme Court, and continues to believe that states should make their own decisions when it comes to the issue of marriage.
Still no word from Clinton, who's known as a longstanding ally of LGBT groups.
--Dana Goldstein
Update: Apologies -- although the Clinton campaign has not released a press release on the ruling, they have made the following statement to the Associated Press:
[Clinton] believes that gay and lesbian couples in committed relationshipsshould have the same rights and responsibilities as all Americans andbelieves that civil unions are the best way to achieve this goal. Aspresident, Hillary Clinton will work to ensure same-sex couples haveaccess to these rights and responsibilities at the federal level. Shehas said and continues to believe that the issue of marriage should beleft to the states.