Chief U.S. District Judge Vaughn Walker's decision Wednesday to hold a trial on whether Proposition 8 violates gays' and lesbians' rights to equality under the law was a tactical victory for opponents of the November 2008 initiative, who say experts and other witnesses can help them show that Prop. 8 was rooted in anti-gay bias.

Opposed to a trial

Sponsors of the ballot measure had opposed a trial, saying legal precedents and studies about parents and children can easily demonstrate that voters had reasonable grounds to add a traditional definition of marriage to the state Constitution.

But their lawyer, Charles Cooper, did not argue against a trial at Wednesday's hearing in San Francisco and said only that he would try to narrow its scope.

Walker scheduled a hearing Oct. 14 on pre- trial legal disputes, which are likely to include the standards for assessing discrimination under the Constitution and the relevance of the campaigns for and against Prop. 8.

The initiative, approved by 52 percent of the voters Nov. 4, defined marriage as the union of a man and a woman, overturning a May 2008 state Supreme Court ruling that gave same-sex couples a constitutional right to marry in California.

It is being challenged in federal court by two same-sex couples represented by Theodore Olson and David Boies, best known as the lawyers for George W. Bush and Al Gore, respectively, in the Supreme Court case that decided the 2000 presidential election.

The gay-rights organizations that challenged Prop. 8 in the state Supreme Court steered clear of the suit initially, reluctant to raise federal constitutional arguments that could reach the U.S. Supreme Court. But they later asked to join the case as co-plaintiffs, which would give them a voice in strategy and possible appeals.

Olson and Boies opposed the request, accusing the organizations of trying to usurp their case. Walker denied intervention Wednesday, saying the original plaintiffs could argue the case and that adding parties would only slow the proceedings down.

He also rejected intervention on the defense side by the Campaign for California Families, which had tried to qualify an initiative to outlaw same-sex domestic partnerships as well as marriage and told Walker that the official sponsors of Prop. 8 had made too many concessions to gay-rights advocates.

S.F. can participate

But the judge allowed intervention by San Francisco, which had also challenged Prop. 8 in the state court. He said the city brings a unique perspective to the case, with its claims that denying marriage to same-sex couples leads to higher government costs in health care and social services.

Walker stressed, however, that Olson and Boies would handle the bulk of the case against Prop. 8 and that the city should cooperate with them.

E-mail Bob Egelko at begelko@sfchronicle.com.

This article appeared on page D - 3 of the San Francisco Chronicle