Before people start thinking there is an innately male or female brain, please remember that environmental conditions including social ones influence the way the brain is structured due to neuroplasticity.
And think about how boys are encouraged to eat and girls are encouraged to watch their weight, and how child body growth relies on building up periodic fat reserves in order to happen, and you need to eat a lot of protein when building muscle mass, and what the physical implications for that are and how it contributes to our social definition of sex. And how that contributes to cis women being more vulnerable to physical violence.
Some figures claim that 1.7 percent of the population is some form of intersex, which is more common than having red hair I believe.