I just finished reading a really interesting book called Born of a Woman by Bishop John Spong (Episcopalian Church) and his theory is that the virgin birth narrative was added much later to justify the claims of divinity. Apparently it was common in the Roman Empire for important people like kings to have birth narratives added to their biographies to show how they were destined for greatness. Since much of the Jesus story lampoons the emperor stories, it could be a valid theory.