This is not just a linguistics problem but also envolves the early Patristics, some Church history, and some sense of chronologies and the early Councils of the Christian Church and the contexts in which these appreciations developed and their codification into creeds, dogman, and doctrines. That is, more than we can go into here. I'll try an answer in a squash shell.
The Greek we are dealing with here is not Ancient, or Classical Greek, but rather the ''common'', or koine Greek of the first and second centuries AD. Moreover, while the New Testament's original language is koine Greek, that is not the original language of Isaiah. The Septuagint is for the Old Testament a translation into Greek from the original Hebrew.
The Greek word, as best I can determine, did mean 'virgin'. But the verse in Isaiah 7:14 that you cite uses the Hebrew word almah. Now -- Hebrew scholars please correct me if I err here. As I understand this, the word almah means 'young woman' and is in fact noncommital on whether the referant might or might not be a ''virgin''. That is the word that is translated into Greek as parthenos. However, there is a Hebrew word bethulah that means 'virgin' more clearly and specifically. Sooo.... the King James translation ''Behold, a Virgin shall conceive and bear a son....'' is overspecific. It would be better to read ''Behold, a young woman shall conceive and bear a son....''
If one look at that whole section of Isaiah that contains that, Isaiah was evidently trying to bolster King Ahaz' morale and urge him to hold his neutrality and not get envolved with Israel's war against Assyria.
The notion of a virgin birth for Jesus actually came relatively late -- earliest Christianity concentrated on the Resurrection-- as does Eastern Orthodox Christianity to this day. In those days, the idea of a deity taking human appearance and form was not particularly unusual and some were arguing that Jesus was only an apparition -- an appearance of human form but really not human and the nails didnt really hurt. The sense of the Christian experience and Church was that Jesus was really a human and the doctrine of the virgin birth was intended at the time to underscore the humanity of Jesus of Nazareth, called the ''Christos'', a Greek translation of ''Messiah''. On the other hand, the Church had to deal with Arian and other unitarian teachings and so called Mary Theotokos, 'God bearer' -- a doctrine that is really not about Mary but rather about Jesus.
It is perfectly possible to have a doctrine of literal virgin birth without Isaiah 7:14, which seems to have a local meaning, whatever additional foretelling meaning it may or may not have had for events to occur several hundred years subsequently. On the other hand, the doctrine of Parthenogenesis was and is not primarily, if at all, about biology. It is rather about relationships, and the Cappadocian Fathers, especially Gregory of Nazianzus, held that the Christian sense of God was that He is relational.