Two California men challenging a ban on same-sex marriage on Monday said they had been a couple for nine years and felt like third-class citizens, leading them to launch the federal case which could set national precedent.

Same-sex couples Paul Katami (L), Jeff Zarillo (2nd L), and Kris Perry (2nd R) and Sandy Stier pose for photographs before the start of their trial in San Francisco, California January 11, 2010. California's ban on gay marriage goes to trial on Monday in a federal case that plaintiffs hope to take all the way to the US Supreme Court and overturn bans throughout the nation. [Agencies]

The men and a lesbian couple unable to marry in California hope to take their case against the state's Proposition 8 ban on same-sex marriage all the way to the US Supreme Court and to overturn bans throughout the nation.

A loss in the top court, two ranks above the action in the case which began on Monday, would seriously undermine efforts to win gay marriage rights in state courts.

The United States is divided on same-sex marriage. It is legal in only five states, though most of those, and the District of Columbia, approved it last year.

Approval of Prop 8 in November 2008 was a sweet victory for social conservatives in a state with a liberal, trend-setting reputation, and maintained the steady success they have scored on the issue at the ballot box. Where it is legal, gay marriage has been championed by courts and legislatures, not voters.

"I don't think of myself as a bad person," said Paul Katami, describing the persecution he felt from a media campaign warning California parents to 'protect' their children by voting against same-sex unions in the 2008 poll.

He and his would-be husband, Jeffrey Zarrillo, described slights in gay life that ranged from being pelted with rocks and eggs in college to the awkwardness of checking into a hotel and not being able to clarify the relationship.

"Being able to call him my husband is so definitive," Katami said. "There is no subtlety to it. It is absolute."

Gays and lesbians have nearly equal rights under domestic partnership laws, but the two men said that left them feeling second- or third-class citizens and they wanted to be married to have kids.

"We hear a lot of 'What's the big deal?'. The big deal is it is creating a separate category for us," Katami said.

Gay rights lawyers in the case describe their battle as a continuation of the fight against racist laws stopping whites and blacks from marrying. Marriage is a fundamental Constitutional right, and in addition gays and lesbians deserve special protection from discrimination, they say.

The lawyers defending the ban say millennia of tradition limit marriage to heterosexual couples and that a state, without malice, can be cautious about changing the institution. Heterosexual couples can procreate, which society needs to continue, they add.

On Monday, District Court Chief Judge Vaughn Walker peppered lawyers with questions even before they had made their cases. He asked lawyer Ted Olson, arguing in favor of same-sex marriage, whether the institution of marriage had improved as it changed over the years, and why the court should get involved in the case at all, as voters and legislatures act.

"We wouldn't need a Constitution if we left everything to the political process," replied Olson, who won President George Bush his presidency in 2000 in a case against Al Gore argued by David Boies - now working with Olson on this trial.

Charles Cooper, the chief lawyer defending Proposition 8, began by saying ban was passed legally in a state which gives broad support to gays.

"This speaks not ill will or animosity toward gays and lesbians but a special regard for this venerable institution," he said.

US President Barack Obama became a turning point early in the trial. When Judge Walker asked for evidence that changes in marriage law had improved the institution, Olson replied, "The President of the United States."

Obama's parents, a black man and a white woman, would not have been allowed to marry in Virginia before a Supreme Court decision allowing interracial unions in 1967.

Walker later asked for a response from Cooper, who said, "The limitation of marriage to a man and a woman is something that has been universal. It has been across history, across cultures, across society. The loathsome restrictions based on race are of an entirely different nature."

The trial is expected to last about two weeks. The Supreme Court on Monday halted plans to show video of the trial on the Internet, though it will consider the case more fully by Wednesday, when its stay ends. Walker agreed on Monday to allow video recording to continue in the meantime, in case the top court relents.