Dr. Albert Mohler gives an outstanding answer to this important question, and traces the history of significant liberal theologians who have chipped away at the biblical position. Excerpt:
Can a true Christian deny the virgin birth? The answer to that question must be a decisive No. Those who deny the virgin birth reject the authority of Scripture, deny the supernatural birth of the Savior, undermine the very foundations of the Gospel, and have no way of explaining the deity of Christ.
Anyone who claims that the virgin birth can be discarded even as the deity of Christ is affirmed is either intellectually dishonest or theological incompetent.
Read the whole thing.