A judge ordered an Orange County man to continue paying $1,250 a month in alimony to his former wife — even though she's in a registered domestic partnership with another woman.

State marriage laws say that alimony ends when a former spouse remarries. So Ron Garber thought he was off the hook for the payments when he learned his ex-wife, Melinda Kirkwood, registered her new relationship under the state's domestic partnership law.

But an Orange County judge decided that a registered partnership is cohabitation, not marriage, and that Garber must keep writing checks to his ex-spouse.

The case, which Garber said he plans to appeal, highlights gaps between the legal status of domestic partners and married couples, an issue the California Supreme Court is considering as it ponders whether to legalize same-sex marriage in the state.

Garber knew his former wife was living with another woman — and had taken the woman's last name — when he agreed to pay her $1,250 a month in alimony. But he said he didn't know the two women had registered with the state as domestic partners under a law that was intended to mirror marriage.

He said he would not have signed the agreement had his wife disclosed the registered partnership.

"This is not about gay or lesbian," Garber said. "This is about the law being fair."

Lawyers in favor of same-sex marriage are watching the Orange County case and say they will cite it to the state high court as an argument for uniting gay and heterosexual couples under one system: marriage.

The alimony ruling and other gaps in the domestic partnership law "highlight the irrationality of having a separate, unequal scheme" for same-sex partners, San Francisco chief deputy city attorney Therese Stewart said.

A Court of Appeal last year upheld the state's ban on same-sex marriage, citing the state's domestic partners law and ruling that it was up to the Legislature to decide whether gays could wed.

The state attorney general's office has argued that same-sex marriage is not needed because gays already enjoy the rights of marriage under the domestic partners law.