SAN FRANCISCO – A California judge yesterday ruled that the state constitution guarantees gay and lesbian couples the right to marry.

Superior Court Judge Richard Kramer found that the state had failed to show that the ban on same-sex marriages advanced a compelling state interest. He went even further, ruling that California officials could not present a legitimate reason that would provide a rational basis for the law.

In his 27-page decision, Judge Kramer concluded that the law against same-sex marriages amounted to discrimination on the basis of gender. He dismissed claims that tradition alone was sufficient to justify the state’s policy and said such arguments were akin to those advanced against miscegenation decades ago.

“Simply put, same-sex marriage cannot be prohibited solely because California has always done so before,” he wrote.

While state officials and some opponents of gay marriage have argued that the existence of domestic partnerships undercuts the need to permit same-sex marriages, Judge Kramer reached precisely the opposite conclusion.

“The existence of marriage-like rights without marriage actually cuts against the existence of a rational government interest for denying marriage to same-sex couples,” the judge wrote. He said the suggestion that gay couples should be satisfied with the rights of marriage while being barred from the marriage rite itself harkened back to the notion that segregated schools could be separate but equal.

Judge Kramer also said he was unswayed by legal claims that same-sex unions should be banned because procreation is central to the concept of marriage. His opinion cited “the obvious natural and social reality that one does not have to be married in order to procreate, nor does one have to procreate in order to be married.” The judge noted that many members of same-sex couples have produced and raised children.

The judge’s opinion quoted from a Supreme Court ruling two years ago, Lawrence v. Texas, in which the high court struck down an anti-sodomy law as unconstitutional.

Opponents of gay marriage immediately vowed to appeal.

“This is a crazy ruling by an arrogant San Francisco judge,” a social conservative who was a party to the court case, Randy Thomasson, said. Mr. Thomasson, who is president of the Sacramento-based Campaign for Children and Families, noted that Judge Kramer’s ruling essentially invalidated a 5-year-old referendum in which 61.4% of California voters declared that marriage should be defined as a union between a man and a woman.

“Judge Kramer has trashed the people’s vote,” Mr. Thomasson said. “He has done what no judge in America has done: to destroy the vote of the people along with the uniqueness of marriage.”

The court battle was a consolidation of six lawsuits that grew out of the February 2004 decision by San Francisco’s mayor, Gavin Newsom, to issue marriage licenses to same-sex couples. According to the city, 3,955 same-sex marriages were recorded before the California Supreme Court stopped the issuance of additional licenses the following month. Some Democratic politicians have argued that the scenes from San Francisco’s City Hall contributed to a sense of cultural chaos that helped propel President Bush to victory over Senator Kerry last fall.

At a press conference yesterday, Mr. Newsom was clearly elated about the ruling, “We will not be appealing,” the mayor, a Democrat, said wryly. “We’re certainly feeling the judge’s decision is right.”

California’s attorney general, William Lockyer, who is also a Democrat, was initially silent about Mr. Newsom’s decision to ignore state law by issuing marriage licenses to same-sex couples. Ultimately, Mr. Lockyer’s office filed a court challenge seeking to end the practice. A spokesman for Mr. Lockyer said yesterday that the decision was under review.

Gay rights advocates were jubilant about the ruling. “I am ecstatic. It was a wonderful decision,” a lawyer with the National Council for Lesbian Rights, Shannon Minter, said. “I don’t see how the decision could have been any stronger.”

One of the private attorneys who argued to uphold the state law, Matthew Staver, blasted Judge Kramer’s opinion.

“I’m shocked and astounded that a court would find – that a judge would find – there’s no rational purpose to preserve marriage as between one man and one woman,” said Mr. Staver, an attorney with a Florida-based conservative legal group, Liberty Counsel. “Marriage has been a part of our culture and our history for millennia.”

Judge Kramer’s decision was based entirely on the California Constitution. That led to more urgent calls yesterday for the state Legislature to pass a bill putting a constitutional amendment on the ballot. The previous referendum changed only a California statute, not the Constitution. Mr. Thomasson said a petition drive would begin soon if legislators fail to act on the issue, as many analysts expect. “If the Legislature refuses to protect marriage, I think you see something quickly from the voters,” he said.

In May, a Field Poll found 53% of California voters were opposed to same-sex marriage and 43% were in favor of legalizing the practice. However, the same poll found a majority of Californians, 54%,were opposed to a federal constitutional amendment to ban gay marriage.

Ms. Minter said a new referendum to ban same-sex marriage would probably fail. “So much has changed,” she said. “We’re in a completely different place.”

Backers of the proposed constitutional amendment noted that similar measures have passed recently in 17 states. “This decision will throw gasoline on the fire ignited by the pro-marriage movement in California and across the country,” Mr. Staver predicted.

Two challenges to New York’s law against gay marriage are pending before the state’s Court of Appeals. An attorney involved in one of the cases, Roberta Kaplan, said yesterday that Judge Kramer’s ruling could make New York judges more comfortable striking down the state’s policy against same-sex unions.

“I think the judges on the New York Court of Appeals are certainly going to look to it,” said Ms. Kaplan, a partner at Paul Weiss. “A judge is less likely to feel that he or she has to go out on a limb.”

Ms. Kaplan said Judge Kramer’s decision was not unexpected. “I’m not surprised. When you do a constitutional analysis, the arguments on the other side don’t really work,” she said.

Judge Kramer, a former business lawyer in San Francisco, was appointed to the bench in 1996 by Pete Wilson, a Republican governor. The judge’s ruling was labeled a “tentative decision,” but he indicated that he has no plans to reconsider the basic legal conclusions he reached.