I remember reading about why do men have nipples from a book titled THE CALL OF THE TORAH, an anthology of interpretation and commentary on the Five Books of Moses. I can't find the exact passage where it mentions about that but one part, in context of how it is not good that man be alone, explains basically that man was able to reproduce in his androgynous form just as unicellular creatures can as he received the Divine blessing "reproduce and multiply" when in the primitive form.