Objection 1: It would seem that the Mother of God was not a virgin in conceiving Christ. For no child having father and mother is conceived by a virgin mother. But Christ is said to have had not only a mother, but also a father, according to Luke 2:33: "His father and mother were wondering at those things which were spoken concerning Him": and further on (Luke 2:48) in the same chapter she says: "Behold I and Thy father [Vulg.: 'Thy father and I'] have sought Thee sorrowing." Therefore Christ was not conceived of a virgin mother.
Obj. 2: Further (Matt. 1) it is proved that Christ was the Son of Abraham and David, through Joseph being descended from David. But this proof would have availed nothing if Joseph were not the father of Christ. Therefore it seems that Christ's Mother conceived Him of the seed of Joseph; and consequently that she was not a virgin in conceiving Him.
Obj. 3: Further, it is written (Gal. 4:4): "God sent His Son, made of a woman." But according to the customary mode of speaking, the term "woman" applies to one who is known of a man. Therefore Christ was not conceived by a virgin mother.
Obj. 4: Further, things of the same species have the same mode of generation: since generation is specified by its terminus just as are other motions. But Christ belonged to the same species as other men, according to Phil. 2:7: "Being made in the likeness of men, and in habit found as a man." Since therefore other men are begotten of the mingling of male and female, it seems that Christ was begotten in the same manner; and that consequently He was not conceived of a virgin mother.
Obj. 5: Further, every natural form has its determinate matter, outside which it cannot be. But the matter of human form appears to be the semen of male and female. If therefore Christ's body was not conceived of the semen of male and female, it would not have been truly a human body; which cannot be asserted. It seems therefore that He was not conceived of a virgin mother.
On the contrary, It is written (Isa. 7:14): "Behold a virgin shall conceive."
I answer that, We must confess simply that the Mother of Christ was a virgin in conceiving for to deny this belongs to the heresy of the Ebionites and Cerinthus, who held Christ to be a mere man, and maintained that He was born of both sexes.
It is fitting for four reasons that Christ should be born of a virgin. First, in order to maintain the dignity or the Father Who sent Him. For since Christ is the true and natural Son of God, it was not fitting that He should have another father than God: lest the dignity belonging to God be transferred to another.
Secondly, this was befitting to a property of the Son Himself, Who is sent. For He is the Word of God: and the word is conceived without any interior corruption: indeed, interior corruption is incompatible with perfect conception of the word. Since therefore flesh was so assumed by the Word of God, as to be the flesh of the Word of God, it was fitting that it also should be conceived without corruption of the mother.
Thirdly, this was befitting to the dignity of Christ's humanity in which there could be no sin, since by it the sin of the world was taken away, according to John 1:29: "Behold the Lamb of God" (i.e. the Lamb without stain) "who taketh away the sin of the world." Now it was not possible in a nature already corrupt, for flesh to be born from sexual intercourse without incurring the infection of original sin. Whence Augustine says (De Nup. et Concup. i): "In that union," viz. the marriage of Mary and Joseph, "the nuptial intercourse alone was lacking: because in sinful flesh this could not be without fleshly concupiscence which arises from sin, and without which He wished to be conceived, Who was to be without sin."