@SKL I'm not sure why you quoted me as you answered your own question in the second paragraph; to
@Cheshire_Kat. All I'm going to mention here is that Mary and Joseph lived together so maybe it was emphasized that Mary was of marrying age (though I think that was more like 16 back then), or maybe something else. It doesn't matter to me, no offense
עלמה by definition is an unmarried woman and I got into above (which is why I was quoting you) that an out-of-wedlock conception or birth is an inconceivable reading of the text. In the nativity narratives (Matthew 1, Luke 1-2) Mary is described as having been
pledged to marry Joseph, at that point still an עלמה.
Matthew describes how Joseph wanted to λάθρᾳ ἀπολῦσαι αὐτήν (1:19), quietly abrogate the marriage contract (to avoid scandal) once the pregnancy was discovered as he had not had relations with her (πρὶν ἢ συνελθεῖν αὐτούς, "before they came together," Mt 1:18; Mary says ἄνδρα οὐ γινώσκω, "I have not known a man," Lk. 1:34.)
The word ἀπολῦσαι, "put away," typically means divorce but the next verse makes clear they were not yet married, so we are talking about him breaking the contract and returning her to her father's house, in great shame, something that would ruin her life and then some. However, the nature of the pregnancy is revealed to Joseph whereupon he takes her as a wife,
after conception.
The Isaiah prophecy is then explicitly referenced using the term παρθένος, virgin, following the LXX, as the NT almost invariably does (which makes sense as it is also being written in Greek in an at least quasi-Hellenistic milieu.) The prophecy requires the עלמה/παρθένος to
conceive in that state (unmarried or virginal.) If the conception is in the unmarried state, then,
eo ipso, it is a case of bastardy and a shame which is not appropriate for the Messiah either as Isaiah would describe Him or as a Christian reading would have it. If it is referring to conception in a state of virginity, that is inherently supernatural. Now, conception in this state is what is specifically remarkable about the sign, which is how it is described: "The LORD Himself shall give you a sign: Behold, etc." (Is. 7:14) This phrasing is almost always used to indicate the supernatural, thus a supernatural birth heralded by signs and wonders as in the nativity narratives.
You could claim that the Gospel narratives are falsely constructed out of whole cloth. There is really no argument to be made against that within the bounds of the discussion we are currently having. But your specific claim earlier was that they are abusing the Hebrew text by way of "eisegesis", reading into the text what is not there but rather what they would like to be there. For the reasons I have described, though, a close but plain reading of the infancy narratives of Matthew and Luke is entirely compatible with the same type of reading of the Emmanuel prophecy in Isaiah.
Who cares if a woman is a maiden or a virgin.
God.
Certainly Jews (and just about every other society at that point in time and most of history since) put a great deal of value on virginity at the time of marriage as this ensures that a man will be raising his own children. The Deuteronomic law in particular gets into these issues in some detail, occasionally gory detail as in Deut. 22. The virgin birth (and the Catholic dogma of the Immaculate Conception of Mary, that was untainted by original sin in order to form a perfect "vessel" for Jesus) is important as a supernatural sign of the uniqueness of Jesus but also as a sign that he is not strictly of the lineage of man, tainted by original sin.
God Created Man (Adam) then Woman out of Man (Eve) and with the birth of Jesus, Man out of Woman (Mary)
Note that Adam, Eve and Jesus share virgin birth in common.
There's an interesting point here relating to the narrative of the expulsion from the Garden in Genesis 3: God says He will "put enmity between [the snake's] seed and [Eve's] seed." זרעהּ, seed, means in the more literal sense semen and by extension descendants. It is highly unusual, in fact unique, to use this to describe the descendants of a female. This foreshadows the virgin birth.
Except Jesus came out of a vagina and the virgin birth idea was probably added later; not to the text but after the events had happened.
Evidence for this?
Being born of a virgin would disqualify Jesus' patrilineal descent from King David anyway so it doesn't make sense
You're too cute by half here. Joseph was his legal father, otherwise the genealogy wouldn't have been put in Matthew (Luke traces the genealogy through Mary, who was also of Davidic blood.) The virgin birth being a unique phenomenon in history, who is to say that for the purposes of fulfilling the need for Davidic descent is not fulfilled by one or both of these?
unless you look at Roman or Greek polytheism who also had deities with virgin mothers
Some sources have Romulus and Remus as having been conceived by a vestal virgin, for instance, but it is more likely that they imply her actually coupling with Mars. There is the case of Attis, who even more troublingly has some sources saying he was born on 25 December, killed, and resurrected, has no sources for the same until well after the Christian era and also was conceived by crudely natural means—spent semen of a God falling on her. Also the earliest pagan sources for his story date to an era where Christianity and paganism were in conflict and paganism losing badly.
Very few if any of the ancient myths (a) have sources that predate Christianity and (b) involve birth
ex virgo intacta, even if the conception is said to be by miraculous or unusual (other than to put it crudely, penile) means. Looking at the details the supposed cases of pagan virgin births look less and less like that of Christ and more and more like a case of paralellomania.
What's more as to Greco-Roman sources, during and around the time of Julian the Apostate there was a concerted effort to increase the appeal of paganism by aping some aspects Christian belief and practice. Other cultures I am less well versed in but I would be surprised if you could find many parallels that actually stick and are sufficiently ancient.