I dont think they can, the supporters of the prop did not have a leg to stand on In courtJudge overturns California's ban on same-sex marriage.
And now SCOTUS will hear the case. How can they come to any other conclusion than Prop8 is discriminatory?
I think the real question is, will they came back with the narrow division that will overturn it in California, or the broad decision that will have consequences nation wide
I give it 60/40 with a 60% Chance for the narrow result, and 40% for the broader one.
I would prefer the broader decision but will not be disappointed for the narrow.