I was watching a video in Psychology class about why people turn bi or gay.
One scientific fact that got me kinda scared was that a person who is right-handed, has a 1/3% chance to be gay for every older brother he has. (yeah scary)
There is also a theory where scientists believe that the female's system rejects the male chromosome of the boy and so, the boy has chromosomes similar to those of a woman.
Likewise to what @Raflesia said, every human is a female at one point, the ones who produce testosterone will be boys and those who don't, will be girls.