It's not just the pro life thing, he's been socially liberal on many issues and has now decided to change his mind on those. THeres a difference between avoiding the pro life issues and declaring yourself pro choice. Romney declared himself pro choice. It's just like red state democrats on guns, some of them are for gun control but just duck the issue and never deal with it. Some on the other hand say they are pro gun. There is a clear difference.
Romney has been out in front against Gay marriage in an overwhelmingly liberal state. He actively opposed gay Marriage, writing an op ed in the WSJ and testifying before Congress against gay Marriage.
He is not "socially liberal"