LOS ANGELES—A judge ordered a California 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.

California 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.

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