I thought the ruling didn’t necessarily overturn Prop 8. So how is this legally allowed?
Here's the sequence: (1) federal district court in San Francisco holds Prop. 8 unconstitutional; (2) 9th Circuit stays that decision pending appeal; (3) 9th Circuit affirms the district court, but continues the stay pending appeal to SCOTUS; (4) SCOTUS vacates the 9th Circuit decision on jurisdictional grounds (meaning the district court's decision stands, but the 9th Circuit's decision is not precedent anymore); (5) 9th Circuit vacates its stay, meaning the district court decision is now in effect; (6) California state officials start performing gay marriages; (7) pro-Prop. 8 forces ask SCOTUS to reinstate the 9th Circuit's stay, because the SCOTUS decision technically won't be final for another 20 days or so; (8) SCOTUS (per Justice Kennedy, sitting as emergency motions justice) says no.
It dismissed the case for lack of Standing. The Fed. Dist. Ct. ruled Prop 8 unconstitutional. THAT ruling is allowed to stand because SCOTUS said none of the Plaintiffs had standing to appeal it.
In short ... Prop 8 is no more.