SAN FRANCISCO (AP) – California can no longer claim to have a legitimate reason for denying same-sex couples the right to marry, a lawyer for 12 same-sex couples told a judge Wednesday.

Arguing a lawsuit that seeks to overturn the state’s one man-one woman marriage law, Shannon Minter of the National Center for Lesbian Rights said California law already has progressed to the point where assigning same-sex couples anything less than full marital status creates a separate and unequal system that doesn’t pass constitutional muster.

“Even if domestic partnership laws can make all the specific rights and benefits that go with marriage … available, creating a separate class for lesbian and gay couples sends a message that their relationships are less good and worthy of respect,” Minter told a packed courtroom.

Wednesday’s arguments in San Francisco County Superior Court marked the beginning of what is expected to be a drawn-out legal fight in which the city of San Francisco and gay rights groups are seeking to make California the second state where same-sex couples can legally marry. The city and a group of a dozen couples sued in March to overturn the state’s 27-year-old marriage law after the state Supreme Court halted same-sex weddings in San Francisco.

Minter previously said that the state’s domestic partner registry already sets the legal foundation for same-sex marriage. The registry was established five years ago but will expand Jan. 1 with a new law that gives same-sex couples nearly all the rights as married spouses.

It is discriminatory for the state to exclude same-sex couples from marriage if it recognizes they have those rights, he said.

In court documents, the state attorney general maintains the progress the state already has made in advancing gay rights is sufficient to ward off a constitutional challenge. That office is representing the state in trying to uphold the existing marriage law on grounds that it is part of an important tradition.

In court, a city attorney criticized that argument, saying the state merely wants to preserve the status quo.

“Tradition is a nice word. It’s sort of fuzzy and soft and sounds good, but the Constitution requires the state to be more specific. Tradition is not a justification,” attorney Therese Stewart said. “It’s a conclusion.”

Judge Richard Kramer pointed to the common ground.

“It looks like both sides are arguing that the institution of marriage is something special,” Kramer said in court. “One side is saying it can’t be denied to some people. The other side is saying it’s special and can’t be changed.”

Arguments were expected to last all day, and the judge said he would rule sometime after mid-January.

Senior Assistant Attorney General Louis Mauro urged the judge to move cautiously and uphold existing law. He said the separate-but-equal argument doesn’t work in this case.

“Petitioners keep saying that because they know it strikes a chord,” he said. “This isn’t about a separate school or a separate housing development. This isn’t about separate rights. It’s about the same rights and which label applies to that.”

The state contends that if Californians want to legalize same-sex marriage, the way to do it is through the Legislature or a ballot proposition submitted to voters, not the courts.

Supporting that argument are two Christian legal groups opposed to any spousal rights for same-sex couples, the Arizona-based Alliance Defense Fund and the Florida-based Liberty Counsel. The organizations represent two traditional-marriage groups based in California.

They submitted papers in the case showing they want the court to rule that same-sex couples can’t marry because the purpose of marriage is procreation.

At issue in the cases is a 1977 amendment to the California Family Code that defined marriage as “a personal relation arising out of a civil contract between a man and a woman.” Before that, the law had an age limit and other requirements but was silent on the subject of gender.

The trial is the first stop in what is expected to be a yearlong odyssey that could end in the state’s highest court.

The consolidated cases were brought after the California Supreme Court ordered San Francisco officials to stop granting marriage licenses to same-sex couples. At that time, the court indicated it might be willing to consider the core constitutional issue after it passed through the lower courts.

A news conference scheduled during the lunch break grew raucous when lawyers on both sides got in a brief shouting match and half a dozen same-sex couples sang “We Shall Overcome”.