http://www.gotquestions.org/virgin-birth.html
Question: "Why is the Virgin Birth so important?"
Answer: The doctrine of the virgin birth is crucially
important (Isaiah 7:14; Matthew 1:23; Luke 1:27, 34). First, let’s look at how
Scripture describes the event. In response to Mary’s question, “How
will this be?” (Luke 1:34), Gabriel says, “The
Holy Spirit will come upon you, and the power of the Most High will
overshadow you” (Luke 1:35). The angel
encourages Joseph to not fear marrying Mary with these words: “What
is conceived in her is from the Holy Spirit” (Matthew 1:20). Matthew states
that the virgin “was found to be with child through the Holy
Spirit” (Matthew 1:18). Galatians 4:4 also teaches the
Virgin Birth: “God sent His Son, born of a woman.”
From these passages, it is certainly clear that Jesus’ birth was
the result of the Holy Spirit working within Mary’s body. The
immaterial (the Spirit) and the material (Mary’s womb) were both
involved. Mary, of course, could not impregnate herself, and in
that sense she was simply a “vessel.” Only God could perform the
miracle of the Incarnation.
However, denying a physical connection between Mary and Jesus would
imply that Jesus was not truly human. Scripture teaches that Jesus
was fully human, with a physical body like ours. This He received
from Mary. At the same time, Jesus was fully God, with an eternal,
sinless nature (John 1:14; 1 Timothy 3:16; Hebrews 2:14-17.)
Jesus was not born in sin; that is, He had no sin nature (Hebrews 7:26). It would seem
that the sin nature is passed down from generation to generation
through the father (Romans 5:12, 17, 19). The Virgin Birth
circumvented the transmission of the sin nature and allowed the
eternal God to become a perfect man.