The state high court's decision, made in closed conference, does not foreclose future attempts in other courts to stop the marriages, though the chance of any new bid succeeding appeared dim.
The court issued its decision in a brief order that did not explain the justices' reasoning.
Andy Pugno, chief legal counsel for ProtectMarriage, the sponsor of Proposition 8, said the California court's rejection of the group's challenge "leaves grave doubts about the future of the initiative process in our state."