The nation's first trial on same-sex couples' right to marry - and the voters' power to forbid those marriages - begins today in a San Francisco federal courtroom that will serve as a forum for two diametrically opposed worldviews.

For two couples and their allies who have filed a lawsuit seeking to overturn California's Proposition 8, the November 2008 initiative was merely the latest example of historic discrimination against gays and lesbians.

Same-sex marriage poses no threat to opposite-sex couples, children or the public welfare, they argue, and a ballot measure that revoked the marital rights of one "disfavored group of citizens" was an unconstitutional appeal to fear and prejudice.

For Prop. 8's sponsors, a religious coalition called Protect Marriage, anti-gay bias is no longer significant in California, where legislators have legalized domestic partnerships and twice voted to authorize same-sex marriage. Discrimination also had nothing to do with the ballot measure, which merely wrote the time-honored definition of marriage into the state Constitution, they argue.

Extending wedlock to gays and lesbians, they maintain, would radically redefine marriage, weaken biological parents' connection with their children, tell men that "they have no significant place in family life" and force many religious Americans to "choose between being a believer and being a good citizen."

Dueling theories

The competing legal theories that will come up in court are a bit simpler: whether Prop. 8 violates the constitutional guarantee of equal protection by discriminating on the basis of sexual orientation and gender, or whether it validly reserves marital status for those who can naturally conceive children.

The initiative overturned a May 2008 state Supreme Court ruling that allowed gays and lesbians to marry in California. The state high court upheld Prop. 8 in May 2009 in a challenge by gay rights advocates whose claims involved only state law and not the U.S. Constitution.

A few days before the state court ruling, two couples and a recently formed advocacy group, the American Foundation for Equal Rights, sued in federal court. Their lawyers are the unlikely duo of Theodore Olson and David Boies, who represented George W. Bush and Al Gore, respectively, in the Supreme Court case that decided the 2000 presidential election.

Initial unease

Established gay rights organizations had avoided federal court, fearing a possible adverse ruling by a conservative U.S. Supreme Court. But with the fate of same-sex marriage in California, and possibly elsewhere, at stake in the trial, the advocates are all on board and most have filed supportive briefs.

The city of San Francisco, a plaintiff in the state court case, has also joined the federal suit. City officials contend Prop. 8 will increase their costs of caring for couples unable to obtain insurance and will also force San Francisco to deny equal rights to its residents.

The suit was assigned by random draw to Chief U.S. District Judge Vaughn Walker, a 1989 appointee of President George H.W. Bush. Walker, who has gained a reputation as a libertarian and a maverick on the bench, has put his distinctive stamp on the case, while making it clear that he expects his ruling to be appealed.

Walker ordered a nonjury trial and will preside as lawyers question the two couples, Prop. 8 campaign officials, psychologists, cultural researchers and other witnesses over the next two to three weeks - all posted for public viewing on YouTube, the result of another unprecedented Walker order.

Prop. 8's sponsors opposed broadcasting the trial on YouTube and fought Walker's order all the way to the U.S. Supreme Court, arguing that the video display would likely cause "harassment of witnesses, threats to the safety and security of trial participants, unnecessary public exposure and ridicule of trial participants." The high court had not issued a ruling by press time Sunday night.

A trial is needed, the judge said, because critical facts are in dispute - for example, the extent of discrimination against gays and lesbians, the possible effects of same-sex marriage on opposite-sex couples and the intent of Prop. 8.

The measure's intent, which its sponsors insist is legally irrelevant, could prove to be the decisive issue.

The U.S. Supreme Court, where this case may be headed, struck down state laws against same-sex sodomy in 2003. But it has given little indication it would overturn marriage restrictions that exist in most states.

On the other hand, the court declared in 1996 that laws motivated by anti-gay bias were unconstitutional - a ruling that Prop. 8's opponents want to apply to their case.

The ballot measure was "motivated by moral disapproval and irrational views" about gays and lesbians, plaintiffs' lawyers said in court papers.

No hidden agenda

Prop. 8's sponsors - vaulted into the case by state Attorney General Jerry Brown's refusal to defend the measure - say its clear-cut goal was to reinforce traditional marriage, and any inquiry into the campaign's allegedly hidden motives is both intrusive and pointless.

"The traditional definition of marriage does not reflect animus against gays and lesbians," attorney Charles Cooper said in court papers.

"It simply reflects the fact that the institution of marriage is, and has always been, uniquely concerned with promoting and regulating naturally procreative relationships between men and women to provide for the nurture and upbringing of the next generation," Cooper wrote.

The trial will test such assertions, with competing experts arguing about the history and meaning of marriage, the adequacy of domestic partnership as a marital substitute, and the social and political status of gays and lesbians.

Walker has kept his views to himself, but his rulings so far have dismayed some of Prop. 8's supporters, who appear to be bracing their followers for a short-term defeat.

"The consistency with which the judge has sided with our opponents is anything but comforting to supporters of traditional marriage," Andrew Pugno, general counsel for Protect Marriage, said in a letter to backers of the measure last week.

Fortunately, Pugno said, the last word will come from "the nine justices on the highest court in the nation."