In general, a person or organization bringing a lawsuit (called a plaintiff) needs to be ACTUALLY INJURED by the acts (or failure to act) of the person or organization being sued (called a defendant).
Put simply, Jane cannot sue John unless Jane can convince the court that, if what Jane says about what John did is true, then Jane would have been injured by what John did.
The gay marriage case is interesting legally, because it is clear that the plaintiffs had standing, in that the law prevented them, as homosexual people, from getting married, which caused them injury, because they were being prevented from doing something that heterosexuals could do. The defendants may not have had standing to defend the lawsuit, as none of the defendants are being actually injured by the fact that gay people can or cannot get married. The courts do not consider religious or philosophical indignation to be actual injury.
So the plaintiffs could bring the lawsuit, but the State of California refused to defend the lawsuit. The District Court allowed the defense to be brought by the same organizations who funded the proposition that made gay marriage illegal. If the case had not been defended, the plaintiffs would have WON BY DEFAULT.
Now, however, the case is on appeal. The appellants are the defendants in the District Court Case, because the defendants lost the case in the district court.
But it is almost certain that the appellants do not have the legal standing to either defend or appeal the lawsuit brought by the gay people who could not get married.
It is amazing that the 9th Circuit Court of Appeals is asking the CALIFORNIA Supreme Court to advise it on this issue.
My guess is that the California Supreme Court will NOT advise the 9th Circuit. I further guess that the 9th Circuit will find that the appellants do not have standing and the US Supreme Court will agree with the 9th Circuit.
NO COURT wants to make a decision on a controversial issue if there is an easy way out of the problem. This is why lawyers really worry about having messed up some small detail which the court can use to throw the case out.