John Chrysostom

But if, when their mouths are stopped on this point, they should seek another, namely, what is said touching Mary's virginity, and should object to us other translators, saying, that they used not the term "virgin," but "young woman;" in the first place we will say this, that the Seventy were justly entitled to confidence above all the others. …

Homily on the Gospel of Matthew 5 ↗

Theophylact of Ohrid

Behold, the Virgin shall be with child. The Jews say that it is not written in the prophecy "virgin" but "young woman." To which it may be answered that "young woman" and "virgin" mean the same thing in Scripture, for in Scripture "young woman" refers to one who is still a virgin. Furthermore, if it was not a virgin that gave birth, how would it be a sign, something extraordinary? Listen to Isaiah who says, "For this reason the Lord Himself shall give you a sign," and immediately he adds, "Behold, the Virgin" (Is. 7:14). …

Commentary on Matthew ↗

Thomas Aquinas

And this is stated in the prophecy: Behold, a virgin shall conceive; "Like the crocus it shall blossom abundantly, and rejoice with joy and singing" (Is 35:2). Likewise, A virgin shall bear a son, because in bearing, her virginity was not impaired in any way: "There shall come forth a shoot from the stump of Jesse, and a branch shall grow out of his roots." Christ is the blossom; therefore, the virginity was not harmed in any way. He continues, and his name shall be called Emmanuel. …

Commentary on Matthew ↗