People are generally born with mental predisposition toward some form of gender in the male/female continuum. However, it is more due to the environment of your mothers womb. Third sons are significantly are likely to be gay than first sons. You could argue that the womb environment is both nature and nurture, but nurture usually refers to upbringing after birth.
I think that the way children are raised has more to do with their willingness to express their true self rather than conform to a societal standard. My parents always encouraged self expression is far more important than conformance.
Of three sons (I am the middle), I am the only one with feminine tendencies, which were there from a very early age. I was also born with an undescended testicle, which is an indication that I had low testosterone at some point in early development. So, I can confidently say that I was born this way.
Remember, we also start out genderless. I am amazed that divergence to male or female succeeds as often as it does. The brain is far more complex, and has a difficult task. Men are supposed to like masculine things, but choose completely opposite in a sex partner. Correctly developing those conflicting interests seems like a difficult task. It is no surprise that there can be significant variations.
Even more important: Who cares why people are the way they are?
