Supporters of same-sex marriage invoke the state's commitment to equality regardless of gender or sexual orientation, the needs of the children of gay and lesbian couples, the persistence of societal discrimination, and legal rights such as freedom of expression, association and privacy.
In defense of its law, the state cites a cultural tradition far older than statehood, the will of the people as expressed in a 2000 initiative, the steps California has already taken toward equal rights for gays and lesbians, and the power of lawmakers and voters to determine state policy.
Beyond those arguments, groups opposing same-sex marriage want the court to justify the state law on moral or scientific grounds, as an affirmation that limiting matrimony to a man and a woman is best for children and society.
A ruling is due within 90 days. The case combines four lawsuits - three by nearly two dozen couples who want to marry and the fourth by the city of San Francisco, which entered the dispute after the court overturned Mayor Gavin Newsom's order that cleared the way for nearly 4,000 same-sex weddings in February and March 2004.
The suits rely on the California Constitution, which state courts have long interpreted as more protective of individual rights than the U.S. Constitution. The plaintiffs invoke a passage in the 1948 ruling on interracial marriage - the first of its kind by any state's high court - in which the justices recognized a "right to join in marriage with the person of one's choice."
Judge Richard Kramer of San Francisco Superior Court echoed that language in March 2005, when he ruled that the state's ban on same-sex marriage violated "the basic human right to marry a person of one's choice." He also said the marriage law constitutes sex discrimination - prohibited by another groundbreaking California Supreme Court ruling in 1971 - because it is based on the gender of one's partner.
But a state appeals court upheld the law in October 2006. In a 2-1 decision, the court rejected Kramer's findings of discrimination and said California was entitled to preserve the historic definition of marriage while taking steps to protect the rights of same-sex couples who register as domestic partners.
Advocates crowd in
As the case reached the state's high court, the participants and the arguments multiplied. Conservative religious organizations, including sponsors of the 2000 ballot measure that reinforced the opposite-sex-only marriage law, accused the state of making a half-hearted defense of its law and sought to justify it as a pro-family measure. Marriage is for procreation, and children fare best with married fathers and mothers, they argued. They also said the definition of marriage is so deeply engrained in the law that judges have no power to change it.
The coalition of conservative religious groups warned that a ruling against the state law would "fracture the centuries-old consensus about the meaning of marriage."
An opposing assortment of liberal denominations counseled the court against a state endorsement of "the religious orthodoxy of some sects concerning who may marry."
The court also heard from hundreds of organizations representing psychologists, anthropologists and other professions, city and county governments, law professors, businesses, civil rights advocates and social institutions.
Judges and limits
Underlying all the arguments is a debate about the proper role of courts in a democracy, particularly on contentious social and political issues. It's the same question - how far, and how fast, judges should move to correct injustices they perceive in the actions of elected officials - that has confronted jurists pondering such issues as segregation, school prayer and abortion.
The subject was raised with unusual frankness in written arguments by Attorney General Jerry Brown's office, which is leading the defense of the marriage law that Brown signed as governor in 1977.
"One unintended and unfortunate consequence of too radical a change is the possibility of backlash," said Deputy Attorney General Christopher Krueger. Same-sex marriage may someday be legalized in California, he said, "but such a change should appropriately come from the people rather than the judiciary as long as constitutional rights are protected."
Brown said last week he wasn't asking the court to sacrifice principles to politics, only observing that rulings that "ride roughshod over the deeply held judgments of society" can have unintended consequences.
He noted that the court majority swung from liberal to conservative after three of his appointees, including Chief Justice Rose Bird, were unseated in a 1986 election that centered on their votes to overturn death sentences.