Ah, but Prop 22 was seven years ago. The numbers for marriage in Massachusetts were even worse back than they were in California.
Just last year, however, Massachusetts' opposition to same-sex marriage was under 50%. The latest numbers I've seen in California are similar.
Finally, Prop 22 didn't ban same-sex marriage -- merely the recognition of out-of-state marriages.
While you may be correct that the governor might avoid doing the right thing on this issue, it will come back to haunt him. I expect that the state Supreme Court will be ruling on the marriage question shortly after any veto, and the fact that legislators who voted for the bill kept their jobs will underscore that there's little political risk to enforcing the constitution's provisions here.
In short, it'll be law, regardless, within 5 years. The question in my mind is whether the governor is seeking a political career in the future -- whether he wants to go down as George Wallace or not. Fortunately for us, our party's on the "right" side of history.