Judge Stephen R. Reinhardt, writing for the majority, relied heavily on a 1996 majority opinion from Justice Kennedy in Romer v. Evans, which struck down a Colorado constitutional amendment that had banned the passage of laws protecting gay men and lesbians. The voter initiative in California, known as Proposition 8, had done something similar, Judge Reinhardt wrote.

That reasoning, he added, meant that the ruling was confined to California.

“We do not doubt the importance of the more general questions presented to us concerning the rights of same-sex couples to marry, nor do we doubt that these questions will likely be resolved in other states, and for the nation as a whole, by other courts,” he wrote.

“For now,” he said, “it suffices to conclude that the people of California may not, consistent with the federal Constitution, add to their state Constitution a provision that has no more practical effect than to strip gays and lesbians of their right to use the official designation that the state and society give to committed relationships, thereby adversely affecting the status and dignity of the members of a disfavored class.”

The Supreme Court has several options in reviewing the decision. It could reverse it, leaving California’s ban on same-sex marriage in place. It could affirm it on the narrower theory, which would allow same-sex marriage in California but not require it elsewhere. Or it could address the broader question of whether the Constitution requires states to allow such marriages.

A plaintiff in the case, Kristin M. Perry, said she hoped that the justices would answer yes to that last question. “There is nothing more important,” she said, “than a state ridding itself of discriminatory laws that hurt its citizens every day.”

Brian S. Brown, the president of the National Organization for Marriage, said the court should address the broader question but say no. “What’s at stake,” he said, “is whether the Constitution demands a redefinition of marriage and whether states can even vote on this issue.”