If the court was hearing Walker's trial decision, then Hollingsworth v. Perry would have much more potential to be the game changer same-sex marriage advocates hope for. However, the court is hearing Reinhardt's far narrower appellate decision. If upheld as is, it could well simply end the civil union / marriage distinction in those states that have what is essentially same-sex marriage in everything but name. (Those states are California, Delaware, Hawaii, Illinois, New Jersey, Rhode Island, and possibly Nevada and Oregon. I'm uncertain if what the last two have is effectively same-sex marriage in everything but name or not.) It would also effectively strike down the state constitutional provisions in Alaska, Colorado, Mississippi, Missouri, Montana, and Tennessee that prohibit same-sex marriage but allow for the state to adopt same-sex marriage in all but name, but only in that it would free those state governments to adopt same-sex marriage, but it would not require them to. Those states that have broader constitutional or statutory restrictions on same-sex relationships would still have them under a Supreme Court ruling that narrowly upholds Reinhardt's decision.
That seems to me like a very odd and convoluted decision, although it would be an interesting decision to see them tackle the civil union/marriage issue. I don't necessarily accept the premise that the Court is only looking at the specific California situation. They could have easily let the Ninth Circuit's decision stand, which would have been completely justified in light of
Romer. I can't see them taking the case up only to simply affirm the lower court. On the other hand, I also cannot see Justice Kennedy upholding Prop 8. Mostly, I'm puzzled as to why the Supreme Court is taking up this case at all, unless it wants to make a broader ruling in support marriage equality. The standing issue only turns Prop 8 from a substantive strike-down to a procedural one (which would mean Walker's ruling takes effect in California). The Ninth Circuit's decision is not likely to constrain the Supreme Court's options, especially when you consider that Olson and Boies are going to push for a 50-state ruling. That is a different strategy compared with their narrower argument at the circuit court. Why else would the opponents of Prop 8 try to make that case at SCOTUS?