A federal judge in San Francisco refused Wednesday to dismiss a lawsuit challenging California's Proposition 8, the voter-approved ban on same-sex marriage, and ordered a trial on whether the measure denies fundamental rights to gays and lesbians.

Chief U.S. District Judge Vaughn Walker has said repeatedly that a trial is needed to resolve disputes that may determine the measure's constitutionality, so Wednesday's ruling did not surprise either side.

But the breadth of his decision was a boost for gay rights advocates, who argue that Prop. 8 unconstitutionally discriminates against gays, is rooted in anti-homosexual bias and violates the right to marry the partner of one's choice.

Walker left all those issues on the table, rejecting arguments by Prop. 8's sponsors that higher courts had already resolved them. Among the questions to be answered, he said from the bench, is "whether Prop. 8 was passed with discriminatory intent."

Voters on trial?

Lawyers for Protect Marriage, the religious conservative coalition that campaigned for the November constitutional amendment, say the measure had a clear purpose - to restore the traditional male-female definition of marriage. They say opponents' claims of a hidden anti-gay agenda are both unfounded and legally irrelevant.

"Voters who passed Prop. 8 are essentially on trial in this case, accused of being irrational and bigoted," Andrew Pugno, a lawyer for the sponsors, said after the hearing.

Protect Marriage is challenging Walker's order that the sponsors must disclose internal campaign strategy documents, which opponents hope will reveal plans to appeal to voter prejudice against gays.

Review of order

A federal appeals court's decision to review that order would delay the trial, now scheduled to start Jan. 11. It will be the nation's first trial on the validity of a law against same-sex marriage.

Prop. 8 amended the state Constitution to define marriage as the union of a man and a woman, overturning a May 2008 state Supreme Court ruling that allowed gay and lesbian couples to marry. The state's high court upheld the measure in May while allowing 18,000 same-sex couples who married before the election to remain legally wed.

The court's ruling was based on state law and did not address any U.S. constitutional issues. Plaintiffs in the federal case - two same-sex couples, a gay-rights group and the city of San Francisco - claim Prop. 8 discriminates on the basis of sexual orientation and gender and interferes with the right to marry one's chosen partner.

Minnesota case

The initiative's sponsors argued that the U.S. Supreme Court validated bans on same-sex unions in 1972 by rejecting a challenge to a Minnesota law defining marriage as being between a man and a woman. The court did not spell out its reasoning but issued a brief order that said opponents had not raised any substantial federal questions.

But Walker said prevailing legal doctrine has changed since 1972. He cited Supreme Court rulings since then that condemned discrimination based on sexual orientation and gender and overturned state laws against homosexual conduct.

Another central issue is whether Prop. 8 should be judged like laws that discriminate against historically persecuted groups. Courts generally overturn such laws unless government can show a compelling need for them.

Rational basis

Charles Cooper, lead attorney for Protect Marriage, noted that the Supreme Court has never categorized gays and lesbians as a persecuted group, entitled to the same legal protection as racial and religious minorities. That means Prop. 8 must be upheld if voters had any rational basis for approving it, he said.

He cited several grounds for such a finding - the traditional definition of marriage, the ability of opposite-sex couples to conceive children naturally, and the voters' decision to wait and see how same-sex marriages work out in other states before allowing them in California.

Gays' political power

But Walker said federal courts haven't resolved the standard for judging laws that treat heterosexuals and homosexuals differently. The answer, he said, could be determined by testimony on such topics as how much political power gays and lesbians hold and to what degree they are discriminated against.

Even if it's rational for the state to promote marriage among opposite-sex couples, the judge asked Cooper, how would that goal be impeded by allowing same-sex couples to wed?

"The answer is, I don't know," the attorney replied. But he said it doesn't matter, because it's up to Prop. 8's opponents to show that the law is irrational.