Reason(s) for being born as men and women Does anyone have an explanation as to how Buddhism explains the reason(s) as to why and how humans are born as men and women? What governs the gender?
The Aggañña Sutta gives a creation style account of human development. In it it says that humans were originally asexual and only later differentiated and grew sex organs. To quote


  And as they did so, their bodies became coarser still, and the
  difference in their looks became even greater. And the females
  developed female sex organs and the males developed male sex organs.
  And the women became excessively preoccupied with the men, and the men
  with the women. Owing to this excessive preoccupation with each other,
  passion was aroused, and their bodies burnt with lust. And later
  because of this burning, they indulged in sexual activity.


Bear in mind though that these is a feeling that the entire sutta is a satire on the  brahminical claims regarding the divine nature of the caste system.