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 [Vulgate: 'Thy father and I'] have sought Thee sorrowing." Therefore Christ was not conceived of a virgin mother.
Objection 2. Further (Matthew 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.
Objection 3. Further, it is written (Galatians 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.
Objection 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 Philippians 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.
Objection 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 (Isaiah 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."
Fourthly, on account of the very end of Incarnation of Christ, which was that men might be born again as sons of God, "not of the will of the flesh, nor of the will of man, but of God" (John 1:13), i.e. of the power of God, of which fact the very conception of Christ was to appear as an exemplar. Whence Augustine says (De Sanct. Virg.): "It behooved that our Head, by a notable miracle, should be born, after the flesh, of a virgin, that He might thereby signify that His members would be born, after the Spirit, of a virgin Church."
Reply to Objection 1. As Bede says on Luke 1:33: Joseph is called the father of the Saviour, not that he really was His father, as the Photinians pretended: but that he was considered by men to be so, for the safeguarding of Mary's good name. Wherefore Luke adds (Luke 3:23): "Being, as it was supposed, the son of Joseph."
Or, according to Augustine (De Cons. Evang. ii), Joseph is called the father of Christ just as "he is called the husband of Mary, without fleshly mingling, by the mere bond of marriage: being thereby united to Him much more closely than if he were adopted from another family. Consequently that Christ was not begotten of Joseph by fleshly union is no reason why Joseph should not be called His father; since he would be the father even of an adopted son not born of his wife."
Reply to Objection 2. As Jerome says on Matthew 1:18: "Though Joseph was not the father of our Lord and Saviour, the order of His genealogy is traced down to Joseph"--first, because "the Scriptures are not wont to trace the female line in genealogies": secondly, "Mary and Joseph were of the same tribe"; wherefore by law he was bound to take her as being of his kin. Likewise, as Augustine says (De Nup. et Concup. i), "it was befitting to trace the genealogy down to Joseph, lest in that marriage any slight should be offered to the male sex, which is indeed the stronger: for truth suffered nothing thereby, since both Joseph and Mary were of the family of David."
Reply to Objection 3. As the gloss says on this passage, the word "'mulier,' is here used instead of 'femina,' according to the custom of the Hebrew tongue: which applies the term signifying woman to those of the female sex who are virgins."
Reply to Objection 4. This argument is true of those things which come into existence by the way of nature: since nature, just as it is fixed to one particular effect, so it is determinate to one mode of producing that effect. But as the supernatural power of God extends to the infinite: just as it is not determinate to one effect, so neither is it determinate to one mode of producing any effect whatever. Consequently, just as it was possible for the first man to be produced, by the Divine power, "from the slime of the earth," so too was it possible for Christ's body to be made, by Divine power, from a virgin without the seed of the male.
Reply to Objection 5. According to the Philosopher (De Gener. Animal. i, ii, iv), in conception the seed of the male is not by way of matter, but by way of agent: and the female alone supplies the matter. Wherefore though the seed of the male was lacking in Christ's conception, it does not follow that due matter was lacking.
But if the seed of the male were the matter of the fetus in animal conception, it is nevertheless manifest that it is not a matter remaining under one form, but subject to transformation. And though the natural power cannot transmute other than determinate matter to a determinate form; nevertheless the Divine power, which is infinite, can transmute all matter to any form whatsoever. Consequently, just as it transmuted the slime of the earth into Adam's body, so could it transmute the matter supplied by His Mother into Christ's body, even though it were not the sufficient matter for a natural conception.
Source: Summa Theologica (New Advent)