Let me begin by stating up front that I do not think that the Prophet Isaiah intended anything like a virgin birth when he states “Behold, the young woman will be with child and bear a son…” in 7.14
(הִנֵּ֣ה הָעַלְמָ֗ה הָרָה֙ וְיֹלֶ֣דֶת בֵּ֔ן). Neither do I think the first evangelist (Mt. 1.22-23) misused this passage when he wrote “Behold, the virgin shall be with child and shall bear a son…” (Ἰδοὺ ἡ παρθένος ἐν γαστρὶ ἕξει καὶ τέξεται υἱόν). The LXX translator already made the decision at one point to choose a more narrow semantic range for his interpretation of הָעַלְמָ֗ה as ἡ παρθένος. So the evangelist did have a text that provided him with Scriptural “support” for his interpretation of the story of the virgin birth.
As an aside this is something I find interesting. There does not seem to be any indication that the Jews of this era expected a virgin-born Messiah. At least none of which I am aware. I know some detractors from the story of the virgin birth note that there are stories of gods being born from virgins, but I don’t think the stand-alone Christology of the First Gospel is as emphatic about Jesus’ divine characteristics as it is his messianic qualifications. If this is so it would seem that there may have very well been an event that made the evangelist search the Scriptures for some evidence that this was foreseen by the prophets rather than the way it is often presented as the evangelist thought this passage from the Book of Isaiah said something about a virgin-born Messiah and/or deity so the evangelist had to construct a story about Jesus to make it appear as if he fulfilled this prophecy. The immediate context of Is. 7.14 does not demand a virgin-born anyone.
In the original narrative הָעַלְמָ֗ה may or may not have referred to a virgin. What is more important is that there is a certain woman in view who maybe would conceive a son for Ahaz or maybe from Isaiah (it is hard to tell: though 8.3 does have Isaiah fathering a son through a prophetess the child is named Maher-shalal-hash-baz or “swift is the booty, speedy is the prey” which seems to have a different tone than Emmanuel meaning “God with us”). In this context what is important is that the child’s birth signifies a clock is ticking toward judgment upon the kings of Israel and Aram.
The evangelist does note this context, as I wrote yesterday, and I think he applies the judgment of kings to the judgment of Herod. This still does not seem to be a sufficient cause for creating a virgin birth narrative. Rather, it seems to me that there was an event that was told as a virgin birth narrative in the early church that forced the evangelist back to the prophets and he made the connection between the LXX translation and Herod’s death as some sort of “fillfullment” (as Bryan E. Beyer wrote in his book Encountering the Book of Isaiah, p. 74).
It seems to me very straightforward that the young woman’s status as a virgin or not a virgin is a secondary issue lost to history. The Prophet foresaw an immediate sign tied into immediate events related specifically to Judah and their King, Ahaz. Nevertheless, I think this makes the use in Mt. 1.23 all that much more dramatic. Since the evangelist had no internal reason to see Is. 7.14 as applied to Messiah (even with the LXX translation, I do not see any good reason to read this as a messianic text in and of itself), it seems that there is good reason to suppose an external reason.
Now, I know virgin births do not happen. Also, I know resurrections do not happen. That does not mean a virgin birth and a resurrection did not happen! I do not have any problem with the doctrine of the virgin birth as I examine how the evangelist read this text and applied it. Rather, I think it serves as good reason for me to assume that something like a virgin birth occurred which led people to seek for a sign from Scripture that God had foretold such an awkward event!
See also: My post examining another connection between Is. 7.14 and Mt. 1.23 here.