Don't get me wrong, I have a very close relative who has decided that, although born male, wants to identify as female. I am totally supportive, but have pointed out that as they were male until their mid-20's they have no idea what it is like to be a biological female. They haven't had periods, not spent their formative years watching who was behind them when walking home in the dark, keeping their keys in their hand to potentially defend themselves, walking into a pub loo watching who may be following them - I could go on.
My relative acknowledged that. We've since had many discussions on the same vein, all of which have been acknowledged & appreciated that they can't totally understand what it is to be female, but that is the gender they feel comfortable with which I have also had to acknowledge. It was tough, but I love them & have to accept them for who they feel they are.
However I do feel that, as a woman who has always been a woman a man who decides that they feel more comfortable living as a woman is only living as half a woman. As I've said, they don't have to deal with periods, haven't grown up watching their backs, their body language etc etc. My relative gets this.