Originally Posted by Paedantic Basterd

If they don't complete a transition, then we've arrived at this between-states thing we've been discussing, where it's easiest and most appropriate to call them what they want to be called.

As for the why of it... we're not there yet. The research is too new, but genetics and fetal hormones and brain structure are all involved. The debate about whether or not it reinforces stereotypes about femininity/masculinity is also a valid philosophical debate and a subject I can't really comment on.