The real reason for this is that the world (yes, I am generalising here) refuses to recognise people as they present themselves to you.
If I see someone who appears to be a woman, who has a woman's name, presents herself as a woman I have absolutely no need to doubt that anymore than I would doubt that someone who presents as a man, is a man.
If you are happier as a woman, or a man, I am pleased that you can find happiness.
Those who are NOT pleased can fuck off!