The proceedings, which are expected to last two to three weeks, involve a challenge to Proposition 8, the gay marriage ban approved by California voters in November 2008.
Regardless of the outcome, the case is likely to be appealed to the U.S. Supreme Court, where it ultimately could become a landmark that determines if gay Americans have the right to marry.
About 100 people demonstrated outside the federal courthouse. Most were gay marriage supporters who took turns addressing the crowd with a microphone. About a dozen gay marriage foes stood in the back of the gathering and quietly held signs demanding the ban remain in place.
Two hours before trial was scheduled to start, the high court blocked video of the proceedings from being posted on YouTube.com. It said justices need more time to review that issue and put the order in place at least until Wednesday.
Over the weekend, Proposition 8's sponsors sought to block YouTube broadcasts. Chief U.S.
District Judge Vaughn R. Walker, who is overseeing the trial, had approved the plan last week, saying the case was appropriate for wide dissemination because it dealt with an issue of wide interest and importance.
At trial, Walker intends to ask lawyers on both sides to present the facts underlying much of the political rhetoric surrounding same-sex marriage. Among his questions are whether sexual orientation can be changed, how legalizing gay marriage affects traditional marriages and the effect on children of being raised by two mothers or two fathers.
"The case is intriguing, exciting and potentially very significant because it addresses multiple important questions that, surprisingly to many, remain open in federal law," said Jennifer Pizer, marriage director for the gay law advocacy group Lambda Legal. "Can the state reserve the esteemed language and status of marriage just for heterosexual couples, and relegate same-sex couples to a lesser status? Are there any adequate public interests to justify reimposing such a caste system for gay people, especially by a majority vote to take a cherished right from a historically mistreated minority?"
The sponsors of Proposition 8, which passed with 52 percent of the vote, won permission to defend the law in court after Gov. Arnold Schwarzenegger and Attorney General Jerry Brown refused to. The attorney general and the governor are defendants in the case because of their positions in state government.
Lawyers for the measure's backers plan to argue that because same-sex marriage still is a social experiment, it is wise for states like California to take a wait-and-see approach. Their witnesses will testify that governments historically have sanctioned traditional marriage as a way to promote responsible child-rearing, and that this remains a valid justification for limiting marriage to a man and a woman.
While other courts have wrestled with the constitutional issues raised by prohibiting same-sex marriages — the Supreme Court last took a look at the issue 38 years ago — Walker's court is the first to employ live witnesses in the task. Among those set to testify are the leaders of the Proposition 8 campaign, academic experts from the fields of political science, history, psychology and economics, and the two plaintiff couples — Kristin Perry and Sandra Stier, who live in Berkeley, and Paul Katami and Jeffrey Zarrillo, who live in Los Angeles.
Chad Griffin, a political consultant who helped spearhead the lawsuit, said the four were recruited to represent California couples who say they would get married were it not for Proposition 8 because they lead lives indistinguishable from those of other couples, gay or straight, who have jobs, children and a desire for the social stamp of approval that matrimony affords, Griffin said.
"Our story, I think, is pretty ordinary," said Perry, 45, the title plaintiff in the case registered on legal dockets as Perry v. Schwarzenegger. "We fell in love, we want to get married and we can't. It's pretty simple."
The women have been together for almost 10 years and since 2004 have been registered domestic partners, a legal relationship that in California carries most of the benefits and obligations of a full-fledged marriage.