Sure, he could have gone rogue and ignored the Supreme Court's decision, in just the same way Obama is choosing to ignore laws he doesn't like.
I know many conservatives argue just that. But if one supports the point of view that the Executive Branch should selectively enforce laws for political reasons, then basically the Rule of Law becomes irrelevant, and will eventually give way to anarchy.
But if I understand it right there was no Supreme Court direction to Romney to do anything. What he did was illegal and morally wrong.
Then you look at his other actions and you see a pattern of supporting homosexuality.
Romney's appointments to the Mass Supreme Court were extremely liberal. I don't know if they were made before or after the Mass SC ruling, but I wouldn't be surprised if his appointments didn't contribute to that ruling.