The point isn’t that parthenos means virgin.  (There are exceptions to even that: once in the translation of Genesis it’s used to describe a rape victim.)  The point is that parthenos, which normally explicitly meant virgin (and still does, as in parthenogenesis), was substituted for a Hebrew word which does not. 

At the very least, the word betulah is much closer to “virgin” than almah, but it wasn’t used in Isaiah’s prophecy.  The Jewish scholars knew the difference between betulah and almah, and yet they wrote “virgin” in Greek instead of “young woman”. 

This was a really important part to get right; why would they take such a liberty with directions to identify God’s son?  To increase the significance of the prophecy by using the already-ancient plot device of a virgin birth (see Perseus, importantly a Greek figure)?  Because of their own ideas of how God ought to reproduce?  And why don’t Jewish scholars agree with the translation today?

I realise that your immediate reaction to that last one will be that if the Jews accepted the translation they’d have to accept Jesus, and they’re not ready to do that.  Maybe so, but put it this way: what reasons do they give for rejecting it, and are those reasons valid or not?

I always say that if you want to know the problems with the prophecies as they pertain to Jesus, you should talk to a rabbi.  Jews believe the Old Testament prophecies as much as you do, but just don’t think they apply to Jesus.  They’re still waiting for their Messiah.

Specifically the fact that it was used as a translation of the Hebrew “almah”, when in fact it would have been a more accurate translation of “betulah” which was not used?

Excuse the lack of context, but if you know what I’m talking about you won’t need it.