IIRC, it wasn't a law that was passed to allow gay marriage, but rather that a law prohibiting gay marriage was found to be unconstitutional by the California supreme court which allowed gay marriage by implication.
Prop 8 was to get an amendment to Cali's constitution so that it couldn't be ruled unconstitutional.
Which means, unless I'm mistaken, that the people of California voted against gay marriage twice. Kind of depressing.