All I know is that I once taught a kid (aged 12) who was biologically a girl, yet dressed, and clearly thought and acted, as a boy. She was also probably the most unhappy child I ever taught.
I can entirely believe she was very unhappy, and I'm sorry about that. But what does 'thought and acted as a boy' mean? Does it mean she didn't want to have long hair, wear the girls' uniform, use makeup? Does it mean she wanted to play football instead of netball? Was she more direct, tougher than girls are habitually supposed to be?
There are so many accounts of girls like this* who had real difficulties at home, at school and in the wider community because of reactionaries with fixed views about how girls and women should behave and dress. These girls not unnaturally end up thinking 'I'm not like all the other girls, everybody keeps telling me that. I like things boys like. I wish I was a boy'. Nowadays that last bit unfortunately often becomes 'I must really be a boy' and organisations like Mermaids will tell these troubled girls that they're right, they were born in the wrong body, and can be helped to change their bodies.
But the vast majority of these girls eventually came to terms with being female and are now happy, well-adjusted women, often with children. Some of them are lesbians, others are straight, some of them have remained gender nonconforming, others now enjoy dressing up etc etc. Puberty is the crucial stage. It's hell to go through but it's a time when the brain matures as well as the body.
We shouldn't be telling girls and boys with gender issues that they were born in the wrong body. We should be telling them they're absolutely fine as they are, that gender stereotypes are harmful and should be ditched, that they can do what they like as long as it doesn't harm others. The one thing that's fixed is their biological sex. That is never going to be changed, no matter what they do to their bodies.