The suits were filed in March 2004 - the day after the court halted a monthlong wedding spree that took place in San Francisco at Mayor Gavin Newsom's direction.
In the four years since then, gay rights advocates have asked the top courts in six other states to legalize same-sex marriage. A ruling is pending in Connecticut, and New Jersey adopted civil unions as an alternative, but none of the other courts were persuaded.
Massachusetts is the only U.S. state where gays and lesbians can legally marry.
But California, due to its size and liberal leanings, represents a different breed of battleground. Not only is the state home to more gay and lesbian couples than any other, its Legislature and courts have a tradition of being pacesetters on gay issues, lawyers on both sides agree, making the prospect of a win or loss here all the more weighty.
The justices "have all seen cases involving lesbians and gay men over and over again, which is a huge plus for us," said Shannon Minter, legal director of the San Francisco-based National Center for Lesbian Rights. "A lot of the other (state) courts were not familiar with our lives and our families."
California was the first state after Vermont to give same-sex couples the legal rights and responsibilities of marriage by a different name - domestic partnerships.
In the last five years alone, the Supreme Court has decided three high-profile cases dealing with gay and lesbian households - including a landmark ruling holding that even after a breakup gays and lesbians had legal obligations to children they had promised to raise with a partner, whether or not an adoption had been formalized.
Along with its hospitable history with gay issues, the San Francisco-based court was the first in the nation to legalize interracial marriage in 1948, a legacy of which gay rights advocates have reminded the current justices.
While such precedents have some of the couples named in the case cautiously preparing for June weddings, opponents of gay marriage think another quirk of California's political climate will work in their favor: its record of valuing laws approved by voters.
The last time California voters were asked to express their views on gay marriage at the ballot box was in 2000, the year after the Legislature enacted the first of a series of laws awarding spousal rights to domestic partners.
Proposition 22, which strengthened the state's existing one-man, one-woman marriage law with the words "Only marriage between a man and a woman is valid or recognized in California," passed with 61 percent of the vote.
"Proposition 22 stands as what we view as an insurmountable hurdle to all the public policy arguments the other side is making," said Glen Lavy, a lawyer for the Arizona-based Alliance Defense Fund. "It comes down to a democratic principle: Who has the right to define marriage? Is it the courts, or is it the group the Constitution says has the ultimate power?"
Lawyers for the gay couples have asked the court to strike down the voter-approved law as an unconstitutional civil rights violation that domestic partnerships cannot repair. Geoffrey Kors, executive director of Equality California, said his group's recent polling shows that California voters are now evenly divided on the same-sex marriage question.
A trial court judge in San Francisco agreed with gay rights advocates and voided the state's marriage laws in April 2005. A midlevel appeals court overturned his decision in October 2006.
Diane Sabin and her partner of 15 years, Jewelle Gomez, plan to watch the arguments broadcast in an auditorium near the courthouse with friends, family and the other couples who are suing the state for the right to wed.
Observing gay marriage opponents argue during earlier court hearings that she and Gomez are not equal to opposite-sex couples was difficult, Sabin said. By contrast, hearing her lawyers challenge that assumption at every turn was healing.
"It's a way of becoming whole, because we don't start out that way in this country," she said.
The court will have no shortage of opinions to evaluate when it considers the combined cases, which include two countersuits filed by gay marriage opponents. Suggesting its significance on a national stage, the matter has attracted 44 friend-of-the-court briefs from across the country - more than any other before the Supreme Court in recent memory.
Thirty of them were submitted in support of legalizing same-sex marriage. The petitioners include a former state Supreme Court justice, the NAACP, 16 state legislators and 19 cities and counties.
Writing in support of the state attorney general's office and the two groups that want to keep gay marriage from becoming legal in California were more law professors, the Knights of Columbus, a coalition of religious groups, Judicial Watch and several groups with missions of limiting parenthood and marriage to opposite-sex couples.
Former Whitewater independent counsel Kenneth Starr, now dean of Pepperdine University's law school, prepared a brief for the Church of Jesus Christ of Latter-Day Saints, the California Catholic Conference, the National Association of Evangelicals and the Union of Orthodox Jewish Congregations of America.
The case also has produced one of the odder alliances of modern California politics. Urging the court to uphold the status quo are Jerry Brown, the state's Democratic attorney general, and Republican Gov. Arnold Schwarzenegger.
As governor in 1977, Brown signed a law that for the first time explicitly defined marriage as a union between a man and a woman. His liberal allies have taken pains to remind him of that decision.
Although Brown now officially serves as the state's lawyer, Schwarzenegger hired his own attorney because of disagreements over whether the attorney general was mounting a vigorous enough defense, an administration source said.
The court has 90 days in which to rule after Tuesday's hearing.
Two groups already are gathering signatures to put an initiative on the November ballot that would extend the current same-sex marriage ban to the state Constitution. If such a measure qualifies and is passed by voters, it would override a ruling by the justices legalizing gay marriage.
