Proposition 8 was narrowly passed by state voters in November, amending the California Constitution to ban same-sex marriage.
It's a legal question profoundly important in California, but being watched all over the country very closely as well.
The justices entered the courtroom and wasted no time getting to the heart of the issue.
"Would you explain to the court why that is a revision to the constitution rather than a mere amendment?" said Justice Kathryn Werdegar from the California Supreme Court.
Opponents of Prop 8 contend it revised the constitution, something only state lawmakers or a constitutional convention can do. They argue voters cannot take away the rights this same court granted last may allowing same sex couples to marry.
"An amendment has to be consistent, a measure that is consistent with the existing purposes, principles and structure of our constitution. A revision is a measure that substantially changes the existing purposes," said Shannon Minter from the National Center for Lesbian Rights.
Justice Joyce Kennard sharply questioned limits on the power of the people.
"The people are those who created the constitution. I think what you're overlooking is the very broad power of the people to amend by initiative the constitution," said Justice Kennard.
Outside the court, the debate was even more spirited, as hundreds on both sides demonstrated, including same-sex couples who legally wed before Prop 8. The court is considering what happens to those marriages.
"Is it really fair to the people who depended on what this court said was the law, is it really fair to throw that out," said Justice Ming Chin from the California Supreme Court.
Kenneth Starr, known most notably for his investigation of former president Bill Clinton, argued for banning gay marriage.
"I don't believe Prop 8 and we've submitted this, 'invalidates,' but it does deny recognition," said Starr.
"Almost certainly the 18,000 marriages from before are safe. Nobody was interested in the retroactivity argument," said ABC7 legal analyst Dean Johnson.
Johnson thinks Prop 8 may be overturned by what he calls a 'shaky four-three majority.' We will know for sure in about 90 days.