There is a more glaring problem here: "παρθένος" here can be translated as either "young woman" or "virgin", with the former being the more common original meaning. The virginity of Mary can, in fact, very well be a striking mistranslation that's stuck over the centuries.
I can't edit this anymore but I'm able to elaborate this a bit more. Matthew repeatedly makes a point of quoting OT scripture to say that Jesus fulfilled it. One of the lines of scripture he quotes is Isaiah 7:14, the one we're talking about. In the Hebrew it says "maiden," but the Greek (septuagint) translation said "virgin." Matthew seems to take it as "virgin," given that in Matthew 1 he has the angel coming to Joseph and telling him not to divorce Mary because Jesus is born of the Holy Spirit, quoting Isaiah 7:14.
> [...] ἡ παρθένος ἐν γαστρὶ ἕξει καὶ τέξεται υἱόν [...]
There is a more glaring problem here: "παρθένος" here can be translated as either "young woman" or "virgin", with the former being the more common original meaning. The virginity of Mary can, in fact, very well be a striking mistranslation that's stuck over the centuries.