If it fails, opponents of same-sex marriage will no longer be able to
say that activist judges have imposed their mores on an unwilling
Californian public.
They will anyway! Logic and reason are not the strong suit of these people.
They shouldn't argue it NOW - after all, it is, in fact, the JOB of judges to provide decisions on questions put before them (assuming they have jurisdiction). And those decisions are supposed to be based on legal reasoning, precedent, constitutional principles, and the judges' judgment (that's why they're called judges!) The will of the people matters, but in an interpretive sense to tangential or related statutes, not in terms of the question at hand.
In other words, the people's will and intent matters when it comes to the meaning of the equal protection clause enacted earlier, and other issues. It doesn't directly matter what they previously thought about legal gay marriage, since whether banning it is unconstitutional or not is actually the question at hand and it is not directly addressed in the constitution. If the people's will mattered in that case, no court would ever overturn any legislation, even if it is a completely clear violation of the constitution.
It sounds downright unamerican to say "the people's will doesn't matter" and I don't mean it broadly, but only in the narrow sense that the people's will about a specific constitutional question is irrelevant while it is being decided by the judicial branch. That's WHY the judicial branch exists.
The "activist judge" whine is just a convenient excuse for the losing side when the status quo is not upheld.