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.
