Just adding to what has already been posted, this argument was made as early as 200AD in the work of an anti-Christian polemic by Celsus. Origen, in his book responding to those arguments, also comes to the conclusion that the original Hebrew indicates "virgin", and adds that in context "virgin" is the only meaning which makes any sense. If you read that passage in Isaias, the dialogue between the king and God, God states that He will show the world a great sign, something beyond the boundaries of nature. "A sign to the depths and the heights" is I think the phrase. And God says that the sign shall be the conception (this is the verse). Now, you can see already the problem. If the word just meant "nubile/young woman", well what's so special about that? Women conceive and bear sons every day of the year, that's not the remarkable, earth shattering sign we're told to expect.
This approach is also useful for showing biblical evidence for Mary's perpetual virginity in the New Testament for the sake of our protty friends.
I had fun with this one with my wife back when we were just engaged.
In talking about this I asked my beloved, "Suppose an angel were to appear to you and -- after the "be not afraid" bit -- he told you that you were to going to give birth to a great saint. Now... you would known exactly how that child would be conceived."
Now Mary's response to Gabriel must seem most bewildering to protties, especially when juxtaposed with Zechariah's response and the angel's reaction to both. To any honest, reasoning mind, this is proof of Mary's holy intent to remain a virgin for her entire life.
Yeah, don't trust the Jews to interpret scripture -- especially prophecy related to Christ. If anyone in history had an ax to grind... it's them.