God certainly didn't MAKE Mary pregnant.  You attribute a human act to God?  In Luke's Gospel, the Angel Gabriel asked Mary if she would be willing to be the Mother of the Savior, thereby respecting her freedom to assent or dissent.  There was no visitation in human form to MAKE Mary pregnant.  If you believe in God, and accept that He can do anything regardless of our capability of understanding His actions, then you have your answer.  If you don't believe in God, however, I respect that.