SAN FRANCISCO—California’s Supreme Court declared that gay couples in the nation’s biggest state can marry—a monumental but perhaps short-lived victory for the gay rights movement Thursday that was greeted with tears, hugs, kisses and at least one instant proposal of matrimony.

Same-sex couples could tie the knot in as little as a month. But the window could close soon after—religious and social conservatives are pressing to put a constitutional amendment on the ballot in November that would undo the Supreme Court ruling and ban gay marriage.

“Essentially, this boils down to love. We love each other. We now have equal rights under the law,” declared a jubilant Robin Tyler, a plaintiff in the case along with her partner. She added: “We’re going to get married. No Tupperware, please.”

A crowd of people raised their fists in triumph inside City Hall, and people wrapped themselves in the rainbow-colored gay-pride flag outside the courthouse. In the Castro, long the center of the gay community in San Francisco, Tim Oviatt wept as he watched the news on TV.

“I’ve been waiting for this all my life. This is a life-affirming moment,” he said.

In its 4-3 ruling, the Republican-dominated high court struck down state laws against same-sex marriage and said domestic partnerships that provide many of the rights and benefits of matrimony are not enough.

“In contrast to earlier times, our state now recognizes that an individual’s capacity to establish a loving and long-term committed relationship with another person and responsibly to care for and raise children does not depend upon the individual’s sexual orientation,” Chief Justice Ronald George wrote for the majority in ringing language that delighted gay rights activists. California has an estimated 108,734 same-sex households, according to 2006 census figures.