SAN FRANCISCO, Calif. (Legal Newsline)-The California Supreme Court will rule Thursday on the legality of the state ban on same-sex marriage, in one of the high court's most closely watched cases this session.

The court in March heard arguments in five consolidated lawsuits challenging the ban on gay marriage, and had until early June to rule on the issue.

The case was brought by nearly two dozen gay and lesbian couples and the city of San Francisco after a judge ruled invalid an order by Mayor Gavin Newsom that allowed nearly 4,000 same-sex couples to marry in 2004.

San Francisco city officials and civil rights groups challenged a state family code law that restricts marriage to a man and a woman.

They also challenged the constitutionality of a 2000 voter-approved ballot measure-Proposition 22-that defines marriage in California as a union between a man and a woman.

In 2006, a San Francisco judge declared the ban unconstitutional, but a state appeals court upheld the law. The judges ruled that it was not the role of judges to legalize gay marriage, but the role of the state Legislature or voters to do so.

California offers same-sex couples who register as domestic partners many of the legal rights and responsibilities once only given to married couples, including medical decision-making and hospital visitation rights.

Arguing before the justices, California Deputy Attorney General Christopher Krueger told the court that the ban on same-sex marriage strengthens state laws on domestic partnerships.

He said domestic partnerships are not exclusionary since they contained all of the legal rights inherent in marriage.

"We submit that when the state is acting so aggressively to protect the rights of domestic partners and families, that it's not irrational to maintain the definition of marriage that has stood the test of time," Krueger said.

From Legal Newsline: Reach reporter Chris Rizo by e-mail at chrisrizo@legalnewsline.com.