It did so by declining to decide a case. For how that worked, see the NY Times:
The case concerning California’s ban on same-sex marriage, Proposition 8, was decided on technical grounds, with the majority saying that it was not properly before the court. Because officials in California had declined to appeal a trial court’s decision against them and because the proponents of Proposition 8 were not entitled to step into the state’s shoes to appeal the decision, the court said, it was powerless to issue a decision. That left in place a trial court victory for two same-sex couples who had sought to marry.