Critics of the law, which grants domestic partners many of the same rights and responsibilities as married couples, said they hoped to get an initiative on next year's ballot to roll back those benefits.
The law grants rights and obligations relating to children, community property, death and other issues to the more than 27,000 couples on the state domestic partner registry. Most are gay men and lesbians, although unmarried heterosexual couples over 65 years old are eligible.
Two conservative legal groups challenged the law as "marriage by another name" and therefore prohibited by Proposition 22, the 2000 ballot measure that affirmed California marriage as between a man and a woman. A lower court denied the challenge, as did a state appellate court.