James, the case before the California Supreme Court was not about other failed attempts by the legislature to legalize gay marriage or about comments by the governor that he would not oppose such measures. The Court had before it a specific statute, the best the elected branches could come up with on the topic. The Court decided they had a better idea, one which the elected branches could not collectively manage to reach. This is the slippery slope of judicial activism that concerns many legal conservatives.
As for John McCain, his aversion to talking about social issues (which some may welcome but social conservatives generally abhor) will certainly ensure that this issue won’t be demagogued on the merits (i.e. marriage). I do think, however, that he is likely to make a broader point, one that appeals not just to conservatives: controversial cultural issues are best not decided by courts.