It all came about from a spelling mistake. The original word was the Hebrew "almah" (young woman) and this was mistranslated into parthenos, meaning virgin. The gospel writer Matthew quoted Isaiah in what seems to be a derivative of the Septuagint version (all but two of the fifteen Greek words are identical) when he said Now all this was done, that it might be fulfilled which was spoken of the Lord by the prophet, saying, 'Behold, a virgin shall be with child, and shall bring forth a son, and they shall call his name Emmanuel.
I checked with my friend the theology prof, and he confirmed that It is widely accepted among Christian scholars that the story of the virgin birth of Jesus was a late interpolation, put in presumably by Greek-speaking disciples in order that the (mistranslated) prophecy should be seen to be fulfilled.
Some modern versions such as the New English Bible correctly give 'young woman' in Isaiah. They equally correctly leave 'virgin' in Matthew, since there they are translating from the Greek."
Quote:Isaiah 7:14 (New International Version, ©2010)
14 Therefore the Lord himself will give you[a] a sign: The virgin will conceive and give birth to a son, and[c] will call him Immanuel.[d]
Footnotes:
1. Isaiah 7:14 The Hebrew is plural.
2. Isaiah 7:14 Or young woman
3. Isaiah 7:14 Masoretic Text; Dead Sea Scrolls son, and he or son, and they
4. Isaiah 7:14 Immanuel means God with us.
Quote: [b]New English Bible
Therefore the Lord himself shall give you a sign: A young woman is with child, and she will bear a son, and will call him Immanuel.