That's what happened in Connecticut a few months ago. Some activist judge (don't know if he's gay or not) decided it was time for CT to "get with the times" and allow gay marriage, basically because "there's greater acceptance of gay marriage" (says who???). We never had a chance vote on it.
I know CT is a Blue State, but I'm not certain that gay marriage would have passed if the people had a chance to vote on it...