I back Saige on this one.
It appears that besides the importance of environment and personal choice, biology has a lot to do with the way we behave. While a fetus develops, there are also brain cells that respond to testosterone, changing development patterns (this is actually technically incorrect - they respond to estrogen, but estrogen can't reach them and there's a whole cyclic thing…). While certain development disorders prevent the creation or totally prevent the reception of male hormones in XY humans, others just prevent some of the effects. Specifically, Persistent Mullerian Duct Syndrome is characterized by the im-prevention of the development of the Mullerian system (duh!) - the female genitalia, but does not prevent masculinization of different brain areas. Whom we desire (if any) is partially our choice, but may be partially dictated by our biology, and not only by which organs we see when we grow up.