SAN FRANCISCO — A judge ruled Monday that California's ban on gay marriage is unconstitutional — a legal milestone that, if upheld on appeal, would open the way for the nation's most populous state to follow Massachusetts in allowing same-sex couples to wed.

The ruling has no effect on Utah, with the recent passage of Amendment 3. It is, however, a moral victory for gay-rights advocates across the country, said Michael Mitchell, executive director for Equality Utah.

Judge Richard Kramer of San Francisco County's trial-level Superior Court likened the ban to laws requiring racial segregation in schools and said there appears to be "no rational purpose" for denying marriage to gay couples.

The ruling came in response to lawsuits filed by the city of San Francisco and a dozen gay couples a year ago after the California Supreme Court halted four weeks of same-sex marriage started by Mayor Gavin Newsom.

The opinion had been eagerly awaited because of San Francisco's historical role as a gay-rights battleground.

Utah is one of 17 states with an amendment defining marriage in their constitutions. California voters overwhelmingly passed a marriage law in 2000 that said, "Only marriage between a man and a woman is valid or recognized in California." Monday's ruling deems that law unconstitutional.

"It takes issue of what marriage means in California away from the people, and one judge announces what marriage really is," said Monte Stewart, president of the Utah-based Marriage Law Foundation.

Such a hot political battle is not likely to find a home in Utah anytime soon, Mitchell said.

"We don't have activist liberal judges in Utah, so I can't really see that Utah anytime soon is going to be overturning Amendment 3," Mitchell said. "Utah is just not fertile ground to do some kind of court case right now."

The state maintained that tradition dictates that marriage should be limited to opposite-sex couples. Attorney General Bill Lockyer also cited the state's domestic-partners law as evidence that California did not discriminate against gays.

But Kramer rejected that argument, citing Brown v. Board of Education — the landmark U.S. Supreme Court decision that struck down segregated schools.

"The idea that marriage-like rights without marriage is adequate smacks of a concept long rejected by the courts — separate but equal," the judge wrote.

It could be months or years before the state actually sanctions same-sex marriage, if ever.

Robert Tyler, an attorney with the conservative Alliance Defense Fund, said the group would appeal. And Lockyer has said in the past that he expected the matter eventually would have to be settled by the California Supreme Court.