In nearly two years since John Lewis and Stuart Gaffney took part in this city's same-sex marriage rebellion, they have filed three rounds of tax returns, buried a mother, vacationed in Europe and attended five weddings — four involving both a bride and a groom.

These are the typical joys, sorrows and rituals of couples who have been together for nearly two decades. Yet for Lewis and Gaffney, one of 4,037 pairs whose San Francisco-issued marriage licenses were invalidated by the California Supreme Court, daily domesticity also brings reminders of the legal rights they do not have and are suing to secure.

"We took a vow at San Francisco City Hall to be together for better or worse," said Gaffney, 43. "It's hard because while we wait, we wait in a separate and unequal status."

A state appeals court in San Francisco on Monday will consider whether a trial judge erred in declaring the state's existing marriage laws unconstitutional.

The First District Court of Appeal is scheduled to hear 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.

Although any ruling is expected to be appealed to the state Supreme Court, advocates on both sides say the stakes remain high at the intermediate court. New York's highest court upheld that state's one man-one woman marriage laws on Thursday, shifting the gay rights movement's focus to other jurisdictions that might join Massachusetts in legalizing gay marriage.

High courts in New Jersey and Washington state are already deliberating cases brought by same-sex couples. But it is California, home to more same-sex couples than any other state, where a ruling could have the most impact.

"California is the most diverse state in the country, and the gay community has achieved a kind of visibility and integration here that is unlike any state," said Jennifer Pizer, a lawyer with the Lambda Legal Defense and Education Fund who is representing gay couples in both California and Washington.

The appeals that will be heard Monday were brought by California's attorney general and two groups opposed to gay marriage. They followed San Francisco Superior Court Judge Richard Kramer's March 2005 ruling 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.

Deputy Attorney General Christopher Krueger said he plans to offer the appeals court the same argument he presented to Kramer: that until lawmakers rewrite the marriage statutes, California should adhere to a traditional definition of matrimony while offering a separate domestic partnership option that confers most of the same rights and benefits to same-sex couples.

"In defending the law, we are saying it's rational to defer to the legislative process in setting policy in such a sensitive area," Krueger said.

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.

Also challenging the trial court's reasoning are the Campaign for Children and Families and the Proposition 22 Legal Defense and Education Fund. They are arguing that Kramer exceeded his when he struck down a 2000 voter initiative that prevented California from recognizing same-sex marriages performed elsewhere.

Mathew Staver, who represents Campaign for Children and Families, said the fact that New York's judges cited marriage's role in promoting procreation makes him optimistic that a California court would accept similar reasoning.

"Having this decision from New York state's highest court is a huge momentum builder for our argument," Staver said. "New York is a big state, it's an influential state, it's not a conservative court. Yet it voted 4-2 to uphold marriage, and it did it on the same rationale we are arguing in California."

Shannon Minter, legal director for the National Center for Lesbian Rights, said he remains confident that California's previous support for gay rights would inform the court's work, despite last week's setback in New York.

Besides offering gay couples domestic partnership rights, the state has led the nation in making it easier for gay couples to adopt children and to be protected from housing and job discrimination.

"If the state believes, as it says it does, that gay people and our families are entitled to the same protections as heterosexuals, there is no justification for denying us the fundamental right to marry," Minter said.

After seeing gay friends legally marry in Massachusetts last year, Gaffney said the passage of time makes him eager to see the issue move forward in his own state.

"It reminded us of what the lawsuit is all about here, what our friends in Massachusetts already have, which is the ability to get married and then get on with their happily married lives," he said.