I'm surprised b/c even in Political Science 101, it is taught that mayors cannot supercede state law. Arnold could still be diplomatic by saying something to the effect of "If the people of CA want this to be the law of the land, then bring it up as a proposition, but for now, the law states that marriage is solely between a man and a woman. Since my administration and I are charged with upholding the law of the land, that is how it will be."
I realize asking for a little backbone is ludicrous, but simply stating what's in your own job description shouldn't be too out there, should it?