U.S. District Judge Vaughn Walker will preside over a two-week trial on a lawsuit in which two couples claim that California's ban on same-sex marriage violates their federal constitutional rights to due process and equal treatment.
The measure was enacted by state voters in 2008 as Proposition 8.
The couples' lawyers argued in a recent court filing that the initiative "is an irrational, indefensible and unconstitutional measure."
Proposition 8 sponsors contend, however, that it is a reasonable way of preserving the traditional definition of marriage and supporting what they say is marriage's central purpose of having children raised by a father and mother.
Their attorneys have written, "The institution of marriage is, and has always been, uniquely concerned with promoting and regulating naturally procreative relationships between men and women to provide for the nurture and upbringing of the next generation."
Walker will decide the case without a jury and is expected to issue a written ruling sometime after the end of the trial.
The case is considered certain to be appealed eventually to the U.S. Supreme Court, after an intermediate stop in the 9th U.S. Circuit Court of Appeals in San Francisco.
A previous five-year battle over same-sex marriage in California centered on the state Constitution and ended in May 2009 when the California Supreme Court said voters had the power to amend the state document to require marriage to be between a man and a woman.
The lawsuit filed in May by couples Kristin Perry and Sandra Stier of Berkeley and Paul Katami and Jeffrey Zarrillo of Burbank took a new tack by claiming violations of the federal Constitution.
The first witnesses to take the stand Monday will be the four plaintiffs. They are due to testify about the harms they allegedly have suffered from not being allowed to marry.
Their lawsuit says the harms include humiliation, emotional distress, stigma and denial of "the personal and public affirmation that accompanies marriage."
Other witnesses to be called to the stand by both sides will be university professors and other experts who will testify about the definition of marriage, its economic value, whether children of opposite-sex marriages fare better and the history of discrimination against gays and lesbians.
Theodore Boutrous, a lawyer for the two couples, said the case is the nation's first challenge to same-sex marriage restrictions to go to trial in a federal court.
In another precedent, the case will also be the first federal trial in western states to be broadcast, under a pilot program approved by the 9th Circuit Judicial Council on Wednesday.
Walker announced last week he will allow broadcast on the YouTube Web site, with video to be recorded by the court staff and posted on YouTube after a delay.
Walker's Federal Building courtroom is expected to be crowded.
But for those who want to see the proceedings live, the court has arranged for overflow seating with a live video feed in the Ceremonial Courtroom at the Federal Building at 450 Golden Gate Ave. and in the library of the 9th Circuit at Seventh and Mission streets.