But what does that MEAN?
Near as I can tell, it's something along the lines of "I think, therefore I am." It's as substanceless as a feeling. They feel like they are the opposite gender and to them that means they are that gender.
Has has much validity as if I, a Caucasian, decided to identify as black and demand that I be considered as such. It is my identity, therefore you must respect that.
In other words, it's a mental disorder.