Genetics is a lot more complicated than the stuff we all learned in grade school (much of which was wrong in any case).  There are XX men and XY women - fact is, a disproportionate percentage of female models are actually XY.  And an XY woman can mature normally, experience menses, become pregnant and give birth to healthy babies.

And it's not a question of nurture - fact is there is no evidence that a person's environment influences either gender identity or sexual orientation.