SAN FRANCISCO — A federal judge Wednesday refused to dismiss a legal challenge to Proposition 8, concluding that the ongoing courtroom battle over California’s voter-approved ban on gay marriage must be resolved in a full-blown trial.

After two hours of legal sparring, Chief U.S. District Judge Vaughn Walker rejected the arguments of Proposition 8 supporters, who maintained that U.S. Supreme Court precedent and a lack of proof of constitutional violations should sidetrack a lawsuit designed to overturn the ballot measure. Instead, Walker, sensing the challenge to Proposition 8 ultimately could wind up before the Supreme Court, wants a trial to develop a full factual record, including forcing Proposition 8 supporters to justify the reasons behind a state ban on allowing gay couples to wed.

One by one, the judge shot down the legal reasons Proposition 8 lawyers presented to resolve the case now and allow the same-sex marriage ban to remain in force. The judge seemed particularly unpersuaded by Proposition 8 attorney Charles Cooper’s chief argument for a state law confining marriage to heterosexual couples — that the state has an interest in protecting “traditional” marriage because of its importance to procreation in society.

“Procreation doesn’t require marriage,” Walker noted, citing statistics showing that a large percentage of children are born out of wedlock.

The judge’s decision sets the stage for a January trial, which will allow same-sex marriage advocates to argue that Proposition 8 violates the equal protection rights of gay couples because it denies them the right to marry.

Backed by a high-powered legal team led by former Republican U.S. Solicitor General Theodore Olson, two same-sex couples sued to overturn the law in the spring, when the California Supreme Court upheld Proposition 8, concluding that it could not trump the voters who approved the measure in November 2008.

The Supreme Court did leave intact an estimated 18,000 same-sex marriages that took place before Proposition 8 went into effect, creating a two-tiered system in California that gay rights advocates say further backs their case. Walker also appeared concerned about denying gay couples the right to wed while thousands of same-sex marriages remain legal in the state. The judge agreed with Olson’s characterization that California now has a “crazy quilt” of options for marriage.

A trial would focus on several key factual questions, including examining the state’s interest in restricting marriage to heterosexual couples and the history of discrimination against gays and lesbians in the United States. The trial also would explore whether Proposition 8 was intended to discriminate against gay couples by stripping away the right to marry, which had been backed in a state Supreme Court ruling in May 2008 striking down a prior ban on same-sex marriage. Proposition 8 effectively voided that ruling.

The only potential obstacle to a trial proceeding in January is a recent appeal of Walker’s order requiring the Proposition 8 campaign to release documents from the measure’s political campaign to the plaintiffs, who say it could support the argument the law was motivated by bias against gays. Proposition 8 lawyers say the order violates the free speech rights of political campaigns and have asked an appeals court to block release of the material. It is not clear whether that appeal can be resolved by January’s trial date.

Contact Howard Mintz at 408-286-0236.