Marriage equality supporters are also asking the Supreme Court to consider whether state gay marriage bans violate the U.S. Constitution.

Defendants in cases challenging bans in Utah, Oklahoma and Virginia have asked or plan to ask the Supreme Court to step in.

On Friday, Virginia Attorney General Mark Herring asked the high court to hear Bostic v. Rainey. The Fourth Circuit Court of Appeals in Richmond on July 28 upheld a lower court's ruling striking down Virginia's ban as unconstitutional.

Herring, who sided with plaintiffs in the case, argued in his filing that the Supreme Court should take the case because “[t]he question presented is vital to a large population of same-sex couples, to their children, and to their fellow Americans who believe that discriminating against gay people is both unfair and unconstitutional. They may fairly call this 'the defining civil rights issue of our time.'”

Herring's filing arrived a day after lawyers for three couples challenging Utah's ban said they agree with defendants in the case, who have asked the Supreme Court to review an appeals court ruling striking down the ban.

(Related: Utah appeals gay marriage case to Supreme Court.)

Now is the time for the Supreme Court to bring certainty to this fundamental civil rights issue of our time,” said Peggy A. Tomsic, a lawyer for the plaintiffs.

Lawyers for the couples said they would ask the Supreme Court to hear the case.

It is unusual for the winning side to urge the justices to review a case and doing so reflects a fundamental shift among proponents of marriage equality, who before 2009 stood clear of federal courts. When the American Foundation for Equal Rights (AFER) filed its case against California's Proposition 8, many activists warned that a loss at the Supreme Court could set back the movement decades.

With mounting victories in dozens of cases filed in the last year, proponents now appear confident that the high court will rule in their favor.