I don't know much about biblical hermeneutics, but wasn't the New Testament written in Greek? Also, based upon Church tradition and history, even if the word didn't explicitly mean virgin, it is certainly understood to mean it with relation to Mary.
It looks like the Greek version is the point where 'almah' got translated as 'parthenos', a word that more specifically means virgin.
It's definitely become a key part of the tradition. I'm now agnostic, but went to a Catholic school growing up, so I know well how much importance is placed on it. I've just always found it interesting how such a core belief of the most popular religion in history might (or might not) have been based on a simple mistranslation.