It's easy to be a Monday morning quarterback. Mitt acted to avoid a fight with the courts.
I think the things I like most about Mitt is his respect for the rule of law, and his ability to pick his battles where it makes the most difference.
I wish he had taken on the courts -- he would have probably lost, but it would have been fun. Of course, doing so would probably have sunk the bill that puts the amendment on the ballot.
Romney's move was bad in the short run, but it forced the issue to a constitutional amendment, which is how it has to be solved in the long run.
If Romney had taken the other approach, it may have driven the liberal legislature to ignore calls for an amendment (claiming it "wasn't needed").