What happens when a person grows up, knows they could have been changed into something they'd rather be, but wasn't? Then they can begrudge their parents for not taking action.
Has this ever happened even once?

Not that it matters, because this argument still doesn't hold. You're going to let parents do this over the chance that the child may wish that he or she was a different gender? I mean c'mon now.. There is no way a parent can know what gender their child will want to be. Should they take blind guesses? If you pick it randomly, it's no different than letting it happen naturally.