The California Supreme Court answers the Ninth Circuit's certified question -- fairly rapidly, even -- and holds that initiative proponents do indeed have standing under state law to defend that initiative.
The opinion is unanimous. It's also correct. The whole point of an initiative is to get around the regular representative process. This would be frustrated if the executive could effectively invalidate an initiative by refusing to defend it in court.
The (much) harder question is whether, notwithstanding state law, there's Article III standing in federal court. Because just because states might permit a defense doesn't mean that the Constitution allows such a defense in federal court. My personal view is that (1) Article III standing should exist, but (2) under United States Supreme Court precedent, it probably doesn't. To put it differently, if I were writing on a blank slate, I'd hold that there's federal Article III standing, but it nonetheless seems to me that such a position can't be reasonably squared with the Supreme Court's decision in Arizonans for Official Language v. Arizona or the dismissal for lack of jurisdiction in Continental Illinois. I might well not have signed onto those adjudications, but having been outvoted, I'd be compelled -- either as a lower court (precedent) and perhaps under stare decisis even in the Supreme Court -- to follow it. Hence no standing.
Which will make it interesting to see what the Ninth Circuit -- and, potentially thereafter, the Supreme Court -- will do. I think the Ninth Circuit did the right thing to certify the state law issue, and if it adopts a neutral adjudication, I think it should dismiss the appeal on Article III grounds, if what you're doing is deciding the case based upon existing precedent. What else happens is less clear. I'm not sure whether the Ninth Circuit will critique Arizonans for Official Language even though they accomplish the same result; I know I would. And reasonable minds may differ about what an Article III dismissal means as a practical matter for the underlying case. Though I don't think that either of these things changes the proper outcome.
What the Supreme Court would then do is even less clear. Because, on the conservative side, you'll have a group of justices hostile to gay marriage who nonetheless agree with strict standing limitations and Arizonans for Official Language. So they'll be inclined to want to find standing in this case only and for that reason to distinguish the case, but I don't think that's a reasonable interpretation of what the Court said in that case (or in the earlier dismissal). Did they get it wrong in those prior cases? Perhaps. But when you say, as the Court did in Arizonans, that this Court has "[n]ever identified initiative proponents as Article-III-qualified defenders of the measures they advocated," well, that seems a pretty good answer to whether initiative proponents are Article III-qualified defenders of the measures they advocated. Including here. Again, I think that's not the right rule, but the Court signed onto it (without dissent), and even if it's dicta and if the prior adjudications on Article III grounds were done in a summary fashion, that nonetheless tells you where the Court has previously stood.
I'm nonetheless quite confident at least some justices would adopt precisely such a view. Sadly. And on the liberal side, there are the exact opposite preferences: not liking strict Article III limits but being pro-gay-marriage. So they're in a quandry as well: They might well like to overrule Arizonans for Official Language, but would rather not choose this case in which to do it.
So it'll be interesting to see what transpires. But so far, things are going as they should.