I'm no legal beagle, so I don't understand how the Court could retroactively legalize an action that was illegal at the time. I don't see why other states would have to recognize the decision of the state Court either.
Other states would not immediately recognize Calif. same sex marriage if the court ruled that way. However, those people will go back to their home states and file lawsuits in those states to try to force other states to recognize same sex marriage from other states. And eventually they want to go to federal court and challenge the Defense of Marriage Act.