The Arizona case was about a state ballot initiative that made English the official state language. After a state employee sued and the district court found that the official-language law was unconstitutional, the state decided not to appeal. So the appeals court let the sponsors of the ballot initiative step in to defend the law. You can see a clear parallel to the Prop 8 situation. But when the case got to the Supreme Court, it was thrown out on other grounds. (The state employee no longer worked for Arizona, so the suit was moot.) Along the way to saying that, Justice Ruth Bader Ginsburg, writing for a unanimous court, expressed "grave doubts" that the initiative sponsors actually had standing. "Petitioners' primary argument—that, as initiative proponents, they have a quasi-legislative interest in defending the measure they successfully sponsored—is dubious because they are not elected state legislators, authorized by state law to represent the State's interests. … Furthermore, this Court has never identified initiative proponents as Article-III-qualified defenders."
This isn't a settled question. Ginsburg's "grave doubts" on behalf of the court aren't the holding of the case, which makes them dicta— the stuff judges say along the way that doesn't bind the lower courts. Still, you can see why Boies and Olson argued that the Prop 8 proponents may not have standing, and why Judge Walker agreed that they had a point.
At the same time, isn't it odd to think that a majority of the voters could pass a law, and then just because the governor and the attorney general don't like it, no one gets to stand up for it on appeal? Especially after they've been allowed to do so at trial? It's an outcome that allows a court challenge to trump voter preferences in a way that just seems undemocratic and out of joint. One of Sullivan's readers argues that to deny the Prop 8 proponents standing would be to get to the heart of the case for gay marriage, because it would show that "their interests"—the interests of the gay-marriage opponents—weren't harmed by Walker's decision. "That is, they lose nothing as a result of gay marriage being legal." It sounds great rhetorically. But it's declaring victory before actually winning on the merits. And to win because your opponent wasn't even allowed to fight is not at all like winning after he fought to the death and you creamed him. Or like winning because he saw your far superior strength, and forfeited. Slate V: Watch Mark Fiore's animated commentary on activist judges: