A large crowd has gathered this morning outside a federal courthouse on Seventh Street in San Francisco, where a three-judge appeals court panel will hear arguments on the constitutionality of California's Proposition 8.

The proposition, approved by voters in 2008, banned same-sex marriage in the state, but a federal judge in San Francisco declared the ban unconstitutional earlier this year. That ruling is now being appealed to the 9th U.S. Circuit Court of Appeals.

More than 100 people, mostly opponents of Proposition 8, rallied outside the courthouse prior to oral arguments this morning.

Same-sex marriage supporters carried signs with messages such as "Life feels different when you're married," and "We all deserve the freedom to marry."

A smattering of opponents also showed up, carrying messages including "Ninth Circuit vs. God's law" and "Homo sex is sin."

Proposition 8's sponsors are appealing U.S.District Judge Vaughn Walker's August ruling striking down the same-sex marriage ban. Walker ruled that the initiative violated the U.S. Constitution's guarantees of due process and equal treatment.

A three-judge panel will hear arguments from both sides this morning and will be asked to decide among the dueling definitions of marriage and the right to marry.

The judges, who were selected randomly, are Stephen Reinhardt, a leading liberal on the court appointed by President Jimmy Carter in 1980; Michael Hawkins, a moderate appointed by President Bill Clinton in 1994; and Randy Smith, a conservative named by President George W. Bush in 2007.

Their decision will be issued in writing at a later date. The losing side is expected to appeal to an 11-judge panel of the 9th Circuit and to the U.S. Supreme Court.

The three-judge panel will devote an unusually long two-hour session to the arguments today.

"At the heart of this case are two competing definitions of marriage," Charles Cooper, the lead lawyer for sponsors of Proposition 8, told the court in a brief filed last month.

The measure's sponsors are appealing U.S. District Judge Vaughn Walker's ruling that struck down California's voter-approved ban on same-sex marriage in August. Walker said the 2008 initiative violated the U.S. Constitution's guarantees of due process and equal treatment.

Two same-sex couples who sued to challenge Proposition 8 define marriage as a union with one's chosen partner.

The original defendants in the couples' federal lawsuit were state officials, including Gov. Arnold Schwarzenegger and Attorney General Jerry Brown, but they declined to defend Proposition 8 or to appeal Walker's ruling.

The couples' lawyers say the U.S. Supreme Court has stated in 14 different cases that "marriage is a fundamental right of all individuals."

One of those cases was the landmark Loving vs. Virginia decision of 1967, when the U.S. Supreme Court overturned Virginia's ban on interracial marriage and said, "Marriage is one of the basic civil rights of man, fundamental to our very existence and survival."

But the Proposition 8 sponsors say the right affirmed by the Supreme Court in all those cases is only a right to heterosexual marriage, because the very definition of marriage is that it is the union of a man and a woman.

Cooper wrote in his brief that the same-sex couples are seeking to introduce gay unions into the right to marry "in a manner utterly inconsistent with history and precedent."

The three judges of the 9th U.S. Circuit Court of Appeals who will hear the case were randomly selected.

They are Stephen Reinhardt, a leading liberal on the court who was appointed by President Jimmy Carter in 1980; Michael Hawkins, a moderate appointed by President Bill Clinton in 1994; and Randy Smith, a conservative named by President George W. Bush in 2007.

Their decision, to be issued in writing at a later date, will not be the end of the case. The losing side can appeal to an 11-judge panel of the 9th Circuit and to the U.S. Supreme Court.

The three-judge panel will devote an unusually long two-hour session to the arguments.

The first hour is designated for arguments on whether the Proposition 8 sponsors and their committee, Protect Marriage, even have a right to appeal, and whether Imperial County has the right to join the case to defend Proposition 8.

In the second hour, attorneys will argue the constitutionality of Proposition 8.

The two couples claim, and Walker agreed, that Proposition 8 violates two different clauses of the Constitution's 14th Amendment.

The first is the due process right that protects fundamental liberties, which the couples say should include their right to marry.

The second is the right to equal protection of the laws. The couples say gays and lesbians were unfairly singled out for exclusion from marriage.

But the Proposition 8 sponsors say that any allegedly differential treatment can be justified if voters had a rational basis for approving the measure in 2008.

That basis, the sponsors contend, is voters' belief that restricting marriage to heterosexual unions promotes "responsible procreation and child-rearing" values that children receive from their married biological parents.

The plaintiffs' lawyers contend that banning same-sex marriage doesn't help heterosexual marriage and does hurt the thousands of children in the state who are being raised by gay and lesbian parents.

"There is no legitimate interest that is even remotely furthered by Proposition 8's arbitrary exclusion of gay men and lesbians from the institution of marriage," attorneys Theodore Olson and David Boies wrote in a brief to the court.

Proposition 8, enacted as an amendment to the California Constitution, provides that "only marriage between a man and a woman is valid or recognized in California."

The lawsuit challenging it was filed by Kristin Perry and Sandra Stier of Berkeley and Paul Karami and Jeffrey Zarrillo of Burbank.

A 12-day nonjury trial held by Walker in January was the nation's first trial on a federal constitutional challenge to a state ban on same-sex marriage. Walker's ruling overturning the initiative has been put on hold during the appeal.

Although there are two more possible layers of appeal -- an expanded 9th Circuit panel and the Supreme Court -- neither panel is obligated take up further appeals. The full appeals court could decide to leave the three-judge panel's future ruling in place, and the Supreme Court could leave the 9th Circuit's final ruling in place.

But Boies, in a telephone news conference on Friday, predicted that the high court will take up the issue of same-sex marriage soon, in either the California case or another lawsuit.

"The question is whether it's this case or the next case," Boies said.