San Francisco ? A judge ruled Monday that California’s ban on gay marriage is unconstitutional — a legal milestone that, if upheld on appeal, would open the way for the most populous state to follow Massachusetts in allowing same-sex couples to wed.

Judge Richard Kramer of San Francisco County’s trial-level Superior Court likened the ban to laws requiring racial segregation in schools, and said there appears to be “no rational purpose” for denying marriage to gay couples.

Supporters of gay marriage march down Market Street in San Francisco. San Francisco County Superior Court Judge Richard Kramer overturned California's ban on gay marriage Monday and said that withholding marriage licenses from same-sex couples trespasses on their civil rights.

The ruling came in response to lawsuits filed by the city of San Francisco and a dozen gay couples a year ago after the California Supreme Court halted a four-week gay marriage spree started by Mayor Gavin Newsom.

The opinion had been eagerly awaited because of San Francisco’s historical role as a gay rights battleground. Gay marriage supporters hailed the ruling as a historic development akin to the 1948 state Supreme Court decision that made California the first state to legalize interracial marriage.

“Today’s ruling is an important step toward a more fair and just California that rejects discrimination and affirms family values for all California families,” San Francisco City Atty. Dennis Herrera said.

Conservative leaders expressed outrage at the ruling and vowed to appeal.

“For a single judge to rule there is no conceivable purpose for preserving marriage as one man and one woman is mind-boggling,” said Liberty Counsel President Mathew Staver. “This decision will be gasoline on the fire of the pro-marriage movement in California as well as the rest of the country.

Last winter, nearly 4,000 gay couples got married after Newsom instructed the city to issue them licenses, in defiance of state law. The California Supreme Court later declared those marriages void, saying the mayor overstepped his authority. But the court did not address the underlying issue of whether the law against gay marriage violates the California Constitution.