Mary accepted the angelic message that she would give birth to a son conceived by the Holy Spirit. She did not choose this pregnancy but she accepted it. Her husband Joseph does not believe her story (do you blame him?). He wants to be faithful to the law and show compassion to his wife by divorcing her privately. He may have chosen Mary to be his wife but he did not choose this pregnancy. Yet, like Mary, Joseph accepted Jesus as his son despite this unexpected event.