Sex differences in brain growth emerge in the womb, study finds

This corroborates other studies, this is likely how transgender folk are born. Something as normal as hormonal changes in the parent after the brain is developed with a particular sex blueprint can mean the body, sexual organs, are developed a la the opposite sex. Of course, the human experience and every aspect of a life is a dot on a scattergraph - some folk display more femme traits and some more masc traits, independent of sex.
Yes, reproduction is important as a species but it's not that important when you have so few natural predators, and one reason humans are special is our ability to be complex.