Therefore the Lord himself will give you a sign: The virgin will be with child and will give birth to a son, and will call him Immanuel.
The story of the virgin birth is central to the Christmas story. This unusual story gives us insight into the hearts of Mary and Joseph.
When told that she was pregnant, Mary did not question the miracle. Instead, she simply asked how it could happen. Joseph also responds well to the news. When he discovers that his fiancé is pregnant (and he knew he wasn’t the father!) he decided to divorce Mary quietly. Not only would this have prevented Mary from public disgrace, it could have even saved her life as the penalty for being caught in adultery was death as we see in John 8.
When Joseph is told that it is ok to take Mary as his wife, he does so at expense to himself. Rumors would spread that he wasn’t the father of Mary’s baby and those rumors survived to the point that a later historian referred to Jesus as the son of a Roman soldier rather than Joseph’s son.
God had a reason for the miraculous birth however. Not only was there the issue of sin being passed through the father, there was also the matter of heredity. Joseph was a descendant of Solomon and had legal right to the throne but God had declared that the line was cursed.
Mary was a descendant of David as well, but not through Solomon. She fulfilled prophecy of David’s heir taking the throne, and Joseph by legally adopting Jesus, gave him legal right to the throne even though his bloodline had been cursed and forbidden from it. Thus the virgin birth was not just an impressive miracle, it was necessary to accomplish God’s plan.