"The resolution of this procedural question does not turn on the substance of the particular initiative measure at issue, but rather on the purpose and integrity of the initiative process itself," Cantil-Sakauye wrote.
Justice Joyce L Kennard, writing separately, said the "integrity and effectiveness of the judicial process require that a competent and spirited defense be presented" when an initiative is challenged.
Otherwise, Kennard wrote, the state's executive branch could "effectively annul voter-approved initiatives simply by declining to defend them."
UC Irvine Law School Dean Erwin Chemerinsky said the ultimate victor in Thursday's decision would not be known until the U.S. Supreme Court ruled.
"If the Supreme Court uses this as the vehicle for holding there is a right to marriage equality for gays and lesbians, then what the California Supreme Court did today will turn out to be a huge victory for gays and lesbians," said Chemerinsky, who contends that Proposition 8 is unconstitutional.
The 9th Circuit is considering other pending disputes over Proposition 8 beside its constitutionality.
ProtectMarriage wants the appeals court to overturn Walker's ruling on the grounds that he should have disclosed he was in a long term same-sex relationship.
Walker, a Republican appointee with libertarian views, did not publicly disclose his sexual orientation until after his ruling, nor did he attempt to hide it. The silver-haired, now-retired jurist often took his partner to bar events in San Francisco.
The 9th Circuit has also scheduled a hearing for early next month on whether to make public the videotape and recordings of the Proposition 8 trial. ProtectMarriage has argued that they should remain sealed. Gay rights groups want them released.
Gay couples challenged Proposition 8 in federal court just days before the California Supreme Court held that it was a valid state constitutional amendment. The state high court had ruled 4 to 3 in May 2008 that the state's marriage ban was unconstitutional, but Proposition 8 reinstated the ban the following November.