He is a girl should be also be accepted
No. By all means, a male person can never be a girl. So, the pronoun must be "she" in this case.
A boy can be dressing up as a girl, or someone simply mistook a girl for a boy and was corrected by the speaker.
This is about grammar and forming sentences, not your narrow mind.
Dressing up as a girl didn't make you a girl, use your brain (cough cough if you have one cough)
Just because you don't agree with people's opinions doesn't give you the right to have a temper tantrum in the comments. Go bug someone else.
A girl is a girl.
She is a girl