Garlic, thank you (Imean that sincerely, not sarcastically!) I had forgotten the most important factor in that the 'Bible' which early CHristians knew and read was (what is called now) the Septuagint, the Greek translation of the Hebrew scriptures, and in the Septuagint, which I've just checked, Isaiah 7:14 does indeed use parthenos. So the thing to bear in mind is that the Septuagint was a Jewish undertaking, complied probably in the 3rd c. BCE after the conquest of Alexander the Great and the Hellenisation of the ancient world, and, screamingly obviously, before the writing of the New Testament. I wasn't eliberately being selecive so as to deceive you, I had genuinely forgotten to factor that in. I've been working all weekend!
So the prophecy quoted in Matthew is the same as the text of Is. 7:14 in the Septuagint, i.e. it is the quotation as Matthew would have known it, as someone who was probably Jewish but raised and steeped in Greek. Again, there is much that could be said here wrt gender identity across Greek and Hebrew cultures, and Egyptian too, the idealisation of virginity in both etc etc, but no doubt that'd be ignored and written off as 'waffle.'
Of course, much remains unanswered by this account - one big thing being the unescapable conclusion that what Isaiah meant in Is. 7:14 is very different to what Matthew means in his citation of it - but that's part of a much bigger question wrt Jewish-Christian identities in the first century, which is, undoubtedly, 'waffle.' Fascinating, wonderful, truthful waffle.
On that note, I don't mean this in an offensive way, but atheism as depicted on this thread causes me one huge problem, which is that to buy into it, I'd have to shut off a big part of my brain. If 'hard evidence' and one-sentence answers are the only admissible...well, evidence, I'd have to seriously dumb myself down to go along with it. Life is complex, identities of all types are complex (and wrt Mary gender and religious identities are part of the picture). I find all this stuff endlessly fascinating and the possibility that God is part of it all, I find fascinating. To say it's all 'made-up stuff' is just so reductive, so flat-earth. Christianity, and Judaism are historic religions and there is a wonderful wealth of history there to be explored and interpreted, with so much wisdom and humanity and love. I'm not saying this to score points, I honestly think that to give that wealth up, I'd come off poorer; to dismiss it all as 'made-up stuff' I'd have to shut down the best part of my brain.