23“Behold, the virgin shall be with child,
and shall give birth to a son.
They shall call his name Immanuel,”
which is, being interpreted, “God with us.”
24Joseph arose from his sleep, and did as the angel of the Lord commanded him, and took his wife to himself; 25and didn’t know her sexually until she had given birth to her firstborn son. He named him Jesus.The issue of Mary being a virgin, as given by Matthew in his reference to Isaiah 7:14, and in Luke careful writing regarding the conception of Jesus demonstrates the extreme important of this event. Matthew does not develop the Isaiah passage in which this prophecy of the Messiah is given, but affirms that the conception of Jesus would be by a virgin as a fulfillment of the prediction of Isaiah.
Let us briefly consider the passage in which this prophecy was given. To demonstrate to the King Ahaz of Judah, and to the people of the southern kingdom, that God would do His will the prophecy moves between a second person singular, referring to the King, and a second person plural, in reference to the people. God offered that the king ask for an extraordinary sign but the King refused. Consequently Yahweh gave His own extraordinary sign, that a virgin would give birth and that this one born would be Immanuel.
The primary points to recognize in the passage is that the sign is an extraordinary one, a miracle. Second, it is directed to the house of David. Third, the child is God with us. The Hebrew word is הָעַלְמָ (almah), which in the Old Testament refers to a young girl of marriageable age, and also a virgin. This is clearly the way the Jewish scholars of the Greek Old Testament (the Septuagint) understand the word, since they used the Greek word for virgin παρθένος (parthenos).
Now to the New Testament. Matthew and Luke understand the Scripture to be speaking of a virgin, and not merely a young woman. Both Gospels carefully guard against any suggestion that there was a human father. Matthew (1:16), at the end of the genealogy, referring to Joseph, uses "of whom" (ἐξ ἧς, ex hes), the Greek pronoun that is feminine singular, therefore it can only refer to Mary, and not to Joseph. Luke makes a point when mentioning Joseph that he is not the father of the child Jesus, since the article does not occur before Joseph's name, setting him apart in the genealogy, and Luke adds the disclaimer regarding Joseph's relation to Jesus with "as was supposed." See at Matt 1:11 the curse on the line from which Joseph comes, so that his son could not physically be an heir. Mary, on the other hand, is also in the line of David, but through Nathan, another son of David.