It was the second time the majority said marriage is between a man and a woman. It's usually the job of the California attorney general to defend state law, but in this case, Kamala Harris refused to do so, even though it went to the nation's highest court.
Harris, a San Francisco Democrat, said Prop. 8 violates the Constitution, and the U.S. Supreme Court has described marriage as a fundamental right 14 times since 1888.
"It's well within the authority vested in me as the elected attorney general to use the discretion of my office to make decisions about how we will use our resources and what issue we will weigh in on or not," Harris said.
So it was up to the traditional marriage supporters to defend Prop. 8 all the way up to the Supreme Court. They argued the initiative process is well within states' rights.
"Our position all along has been that the political process, that means state-by-state, states deciding for themselves, that that's the forum where this debate belongs," said Andy Pugno, the co-author of Prop. 8.
However, part of the Prop. 8 debate is whether traditional marriage supporters have standing - the right to step in on the state's behalf when it refuses to defend a law.
"He has to show that he has some injury that would result if these marriages were to go forward," Harris said. "Our brief and our contention is that he would not be injured."
The Supreme Court has several options. It could uphold Prop. 8 to keep the ban intact. It could also strike down Prop. 8 to allow same-sex marriage.
And there's the path of ruling narrowly. On several occasions, some of the justices indicated they're reluctant in ruling broadly enough that would same-sex marriage legal across the country.
It is also possible that the initiative process that got the same-sex marriage ban out in the forefront in California in the first place will be used to change the justices' decision.
"No matter who wins or loses, both sides are thinking about a future ballot issue, a ballot battle. But it would be very expensive. It'd be very traumatic for this state," said Randy Thomasson, the executive director of savemarriage.com. "Do we really want 'Marriage War 3' on the ballot?"