SAN FRANCISCO – A state appeals court upheld California’s ban on gay marriage Thursday, a critical defeat for a movement hungry for a win after similar losses in two other states.

In reversing the March 2005 ruling of a San Francisco trial judge, the 1st District Court of Appeal agreed with the state’s attorney general, who argued it is up to the Legislature, not the courts, to change the definition of marriage as a union between a man and a woman.

“We conclude California’s historical definition of marriage does not deprive individuals of a vested fundamental right or discriminate against a suspect class,” the court said in a 2-1 decision. “The time may come when California chooses to expand the definition of marriage to encompass same-sex unions. That change must come from democratic processes, however, not by judicial fiat.”

The justices, in their 128-page opinion, noted that California’s ban on same-sex marriage does not discriminate against gays and lesbians because of the state’s strong domestic partner law, which gives registered couples most of the same rights as married spouses in California.

The ruling does not guarantee, however, that same-sex couples will not ultimately be able to get married in California. Gay marriage advocates said beforehand that they would appeal to the California Supreme Court if the intermediate court did not decide in their favor.

“Though we are disappointed, we always knew this issue was going to be decided by the California Supreme Court,” said Molly McKay, a spokeswoman for Marriage Equality USA. “We believe that the California Supreme Court will enforce the constitutional guarantee of equality under the law and strike down the discriminatory barriers denying same-sex couples access to civil marriage.”

Opponents of gay marriage praised the decision.

“This is a victory for the right of the people of California to make fundamental policy decisions through democratic processes,” said Monte Stewart, President of the Marriage Law Foundation, a Utah-based group that opposes same-sex marriage. “It is also a victory for society’s most consequential social institution, and that is marriage.”

In 2004, Massachusetts became the first state to legalize same-sex marriage after gay and lesbian couples in that state successfully sued for the right to wed. In the aftermath of that change, 19 states passed constitutional amendments barring gay marriage. Another 26 have statues limiting marriage to a man and a woman. Connecticut and Vermont allow civil unions.

Advocates of same-sex unions had seen California as one of their best chances to expand their marriage rights after recent high court rulings in New York and Washington state upheld bans in those states.

Thursday’s ruling came three months after the appeals court heard six hours of arguments in as many related cases – four of them filed by the city and lawyers for 20 couples seeking the right to wed, and two brought by groups that want to maintain the status quo barring same-sex unions.

The lawsuits arose out of the 2004 same-sex marriage spree that San Francisco Mayor Gavin Newsom ignited when he instructed city officials to issue marriage licenses to gay and lesbian couples. Hundreds of couples from across the country flocked to City Hall to marry, but groups opposed to gay marriage persuaded the state Supreme Court to invalidate the licenses.

“This is a disappointing second round in what we’ve always known is a three-round fight,” San Francisco City Attorney Dennis Herrera said of Thursday’s ruling.

In March 2005, San Francisco Superior Court Judge Richard Kramer ruled that the state’s existing marriage laws violated the civil rights of gays and lesbians by denying them “the basic human right to marry a person of one’s choice” and by discriminating on the basis of gender and sexual orientation.

Following Kramer’s decision, the Legislature last year became the first lawmaking body in the nation to legalize gay marriage. Gov. Arnold Schwarzenegger vetoed the bill, saying it was up to voters or the courts, not lawmakers, to settle the contentious issue.

With a gubernatorial election next month, Thursday’s ruling is sure to put the issue squarely back in the state’s political arena. The bill’s sponsor, Assemblyman Mark Leno, plans to reintroduce the measure to allow gay marriage.