A June U.S. Supreme Court ruling upheld a trial judge's order striking down the ballot measure as unconstitutional.
On June 28, California Governor Jerry Brown ordered county clerks to begin issuing marriage licenses to same-sex couples.
Prop. 8 supporters then asked the state Supreme Court to stop the weddings, arguing that the federal ruling applied narrowly, and only to the two couples who filed the federal lawsuit challenging the ban.
The state high court had asked for additional written arguments on those issues by Aug. 1.
With little comment Wednesday, a unanimous state Supreme Court allowed gay marriages to continue.
The Associated Press contributed to this report.