It may (or may not) surprise you to hear that many liberal christians have already purged this from their theology, and for good reason. Not only is the virgin birth blatantly miraculous, it is also very weakly supported by the bible and to top it all off it was not unique to christianity but rather the result of many centuries of mingling belief systems around the mediterranean. The greek speaking author of the gospel of Matthew referenced a passage in Isaiah from the Septuagint, which had been influenced by the cult of Ishtar and had replaced the hebrew word for young woman (almah) with the greek word for virgin (parthenos). As you can see this mistranslation of this magnitude could have devastating effects on theology and doctrine. This also provides some explanation as to why only 2 out of the 4 gospels mention the virgin birth, something that isn't exactly a passing matter. The earliest gospel (Mark) doesn't mention the childhood or birth of Jesus at all and the author of John (the last of the 4 gospels) wouldn't dream of reducing his beloved divine 'Logos' to mere flesh and blood by implying Jesus was human and born of a human.
Back to the 'prophecy'....
The verse from Isaiah that Matthew quoted was Isaiah 7:14
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.
As I already pointed out virgin is a mistranslation of the hebrew as a result of the Hellenic pagan influence on the Septuagint, but besides the mistranslation this verse was taken grossly out of context by none other than the author of Matthew himself. Isaiah refers specifically to when and where this prophecy happened, he was speaking to King Ahaz about current events telling him that Syria and Ephraim would not go to war with him. Isaiah was telling Ahaz that a woman who is currently a virgin would soon become pregnant (thus no longer being a virgin) and when this happens political tensions will cease. It is worthy to note that the prophecy never came true, and the wars did occur. So not only is the doctrine of virgin birth based on a mistranslation, but it is also based on a completely unrelated prophecy that never even came true.
The virgin birth of Jesus is based on a mistranslated false prophecy.