Regardless of whether or not they think they are of a different gender, the fact is that most people associate gender pronouns with physical sex. To them -- us -- the proper pronoun to use is one of physiological fact. Bradley Manning is a he. There is no Chelsea Manning, and if there were, he would be a he. These are the actual facts, to me.
Now, if you choose to recognize gender above sex, and accept that everyone gets to dictate to the rest of us what their gender is, then fine, to you, "she" is correct and "he" is incorrect. However, how does it make sense that Manning can decide for himself his own gender, but I cannot decide for myself that my use of pronouns is based on physiological sex?
It's just another example of how the left is not pro-choice at all, they are only pro-choices-they-agree-with.