"If anything this decision in California will help to establish both in theory and fact that marriage equality is a good thing and not a bad thing," McCoy said. "As that notion plays out in California as it has in Massachusetts, for the last four years, that notion could catch on in other parts of the country."

The case was set in motion in 2004 when Gavin Newsom, the mayor of San Francisco, threw City Hall open to gay couples to get married in a calculated challenge to California law. Four-thousand gay couples wed before the Supreme Court put a halt to the practice after a month.

Two dozen gay couples then sued, along with the city and gay rights organizations.

After the ruling was announced, a crowd of people raised their fists in triumph inside San Francisco City Hall, and people wrapped themselves in the rainbow-colored gay-pride flag outside the courthouse. By the afternoon, gay and lesbian couples had already started lining up at to make appointments to get marriage licenses, while in West Hollywood, supporters were planning to serve "wedding cake" at an evening celebration.

"It's about human dignity. It's about human rights. It's about time in California," Newsom, pumping his fist in the air, told a roaring crowd at City Hall. "As California goes, so goes the rest of the nation. It's inevitable. This door's wide open now. It's going to happen, whether you like it or not."

California has an estimated 92,000 same-sex couples, and more than 10 percent of the nation's overall population, which gives the ruling significant heft. The state already offers same-sex couples who register as domestic partners many of the legal rights and responsibilities afforded to married couples, including the right to divorce and to sue for child support.

Ten states now offer some form of legal recognition to same-sex couples — in most cases, domestic partnerships or civil unions. In the past few years, the courts in New York, New Jersey and Washington state have refused to allow gay marriage.

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E-mail: carrie@desnews.com; dbulkeley@desnews.com

Contributing: Associated Press