I don't usually like to add argumentative writeups, but I feel that the original point about the Virgin Mary was badly made by piq and that quijote's writeup, though good, misses the most important point.
The ambiguous translation occurs in Isaiah 7:14, an Old Testament scripture which was translated from Hebrew into Greek around the 3rd Century B.C. There are two Hebrew words which are usually translated as 'virgin' (or 'parthenos' in the Greek) - 'Bethulah' and 'Almah'. Bethulah definitely indicates a virgin, whereas Almah most often means a young girl, depending on context. The word used in the original Isaiah is Almah, and refers in its context to a woman already pregnant, indicating that the 'young girl' meaning was the one intended. I should note that some scholars dispute this, stating that Almah also means virgin, but in a different sense (perhaps a spiritual metaphor?)
The suggestion is not, therefore, that the New Testament itself, upon which the Catholic Church is based, contains any ambiguity - quijote quotes from Luke and Matthew, missing the point that these Gospels would have been specifically written in order to fulfil the Messianic prophecies made in Isaiah. The mistranslation in Isaiah prompted the authors of some of the Gospels to add further legitimacy to Jesus' status as the Messiah by stating that his mother was a virgin.
This mistranslation in fact is disregarded, or not properly addressed, by the Catholic Church, which in its past has been extremely selective about which texts to include as part of its canon and which to consign to the wastebucket of religious history. Not all the Gospels maintain Mary's virginity, but only four are included in the New Testament Bible.
References, arguments and further reading: