Justices considered a series of lawsuits to overturn the ban, which overruled a 4-3 June court decision that briefly legalized same-sex marriage. Those suits claim Proposition 8 was put on the ballot improperly.
Gay rights marches began early this morning in California and groups have planned rallies tonight, preparing to be arrested in a mass demonstration of civil disobedience.
Join the Impact, which has fought to overturn the gay marriage ban, said they would be organizing national grass roots protests in Day of Decision rallies.
Putting a new question on the ballot to legalize gay marriage could take months, and many gay advocates said it might not be viable until 2010 or later.
"We have a lot of educating to do to convince [voters] that gays and lesbians are equal," said Lambda legal's Pizer. "It's very important that people agree on that point, and that the people of California share that belief."
But Focus on the Family's Hausknecht said pro-Proposition 8 groups are already preparing their "talking points" for a new referendum on the issue.
"How gay marriage affects my [tradtional] marriage is entirely the wrong question," he said. "The right question is how is gay marriage going to affect society in general and religious liberties and the rights of conscience?"
"Across the country we've seen the impact on religious freedom, not just same sex marriage but nondiscrimination statutes," he said.
Opponents of Proposition 8 argued that it revised the California's equal protection clause to such a dramatic degree that its sponsors needed the legislature's approval to submit it to voters.
But several justices at a March hearing said they were skeptical of that argument and, in the end, the Supreme Court did not undermine the state's citizen initiative.
Since the passage of Proposition 8, gay marriage has gained momentum around the nation. Iowa, Maine and Vermont have joined Massachusetts and Connecticut in recognizing same-sex couples. Similar proposals are under way in New Hampshire and New York.
The Democratic-controlled California Legislature has twice passed measures to legalize gay marriage, but they were vetoed by Gov. Arnold Schwarzenegger.
"While I believe that one day either the people or courts will recognize gay marriage, as Governor of California I will uphold the decision of the California Supreme Court," Schwarzenegger said today. "Regarding the 18,000 marriages that took place prior to Proposition 8's passage, the court made the right decision in keeping them intact. I also want to encourage all those responding to today's court decision to do so peacefully and lawfully."
With passage of Proposition 8, California amended its constitution to specify that only marriage between a man and a woman is valid or recognized.
In May 2008, the state's Supreme Court overturned a gay marriage initiative. That decision allowed thousands of gays and lesbians to be legally married in that state; gay couples across the state decided not to take their chances, choosing to marry before voters took up the measure.