Why was it necessary for Mary to be a virgin? A lot of emphasis seems to be placed on the virginity of Mary.  (For discussion on that, see this question)

Is there a theological reason why Jesus's birth had to be a virgin birth?  Obviously God could have chosen a non-virgin to be the mother of Christ, so why did God choose a virgin?
It fulfilled prophecy

From the prophecy of
Isaiah 7:14 (NIV)


  Therefore the Lord himself will give you a sign: The virgin will conceive and give birth to a son, and will call him Immanuel.

It provided us a sign so that we will know the Messiah

The Isaiah verse above illustrates that God gave us this gift of a virgin birth as a sign that Jesus was the Messiah.
It illuminates his sinless nature

Jesus is pure without sin.  The virgin birth illuminates this sinless nature.  Had he been born of a man, he would have been born fallen, like the rest of humanity, therefore carrying sin.
It allows for the Trinity

If Jesus had been born of a man, how could he be the "Son of God"?  He could be in the same sense that we all are "sons of God".  However, the virgin birth allows for the understanding that God himself came down to Earth to be the perfect sacrifice.

This opens up a whole realm of doctrine.  If he was just a man, how could his sacrifice been an atonement for everyone's sin?