Objection 1. It would seem that the matter of Christ's body should not have been taken from a woman. For the male sex is more noble than the female. But it was most suitable that Christ should assume that which is perfect in human nature. Therefore it seems that He should not have taken flesh from a woman but rather from man: just as Eve was formed from the rib of a man.
Objection 2. Further, whoever is conceived of a woman is shut up in her womb. But it ill becomes God, Who fills heaven and earth, as is written Jeremiah 23:24, to be shut up within the narrow limits of the womb. Therefore it seems that He should not have been conceived of a woman.
Objection 3. Further, those who are conceived of a woman contract a certain uncleanness: as it is written (Job 25:4): "Can man be justified compared with God? Or he that is born of a woman appear clean?" But it was unbecoming that any uncleanness should be in Christ: for He is the Wisdom of God, of whom it is written (Wisdom 7:25) that "no defiled thing cometh into her." Therefore it does not seem right that He should have taken flesh from a woman.
On the contrary, It is written (Galatians 4:4): "God sent His Son, made of a woman."
I answer that, Although the Son of God could have taken flesh from whatever matter He willed, it was nevertheless most becoming that He should take flesh from a woman. First because in this way the entire human nature was ennobled. Hence Augustine says (QQ. lxxxiii, qu. 11): "It was suitable that man's liberation should be made manifest in both sexes. Consequently, since it behooved a man, being of the nobler sex, to assume, it was becoming that the liberation of the female sex should be manifested in that man being born of a woman."
Secondly, because thus the truth of Incarnation is made evident. Wherefore Ambrose says (De Incarn. vi): "Thou shalt find in Christ many things both natural, and supernatural. In accordance with nature He was within the womb," viz. of a woman's body: "but it was above nature that a virgin should conceive and give birth: that thou mightest believe that He was God, who was renewing nature; and that He was man who, according to nature, was being born of a man." And Augustine says (Ep. ad Volus. cxxxvii): "If Almighty God had created a man formed otherwise than in a mother's womb, and had suddenly produced him to sight... would He not have strengthened an erroneous opinion, and made it impossible for us to believe that He had become a true man? And whilst He is doing all things wondrously, would He have taken away that which He accomplished in mercy? But now, He, the mediator between God and man, has so shown Himself, that, uniting both natures in the unity of one Person, He has given a dignity to ordinary by extraordinary things, and tempered the extraordinary by the ordinary."
Thirdly, because in this fashion the begetting of man is accomplished in every variety of manner. For the first man was made from the "slime of the earth," without the concurrence of man or woman: Eve was made of man but not of woman: and other men are made from both man and woman. So that this fourth manner remained as it were proper to Christ, that He should be made of a woman without the concurrence of a man.
Reply to Objection 1. The male sex is more noble than the female, and for this reason He took human nature in the male sex. But lest the female sex should be despised, it was fitting that He should take flesh of a woman. Hence Augustine says (De Agone Christ. xi): "Men, despise not yourselves: the Son of God became a man: despise not yourselves, women; the Son of God was born of a woman."
Reply to Objection 2. Augustine thus (Contra Faust. xxiii) replies to Faustus, who urged this objection; "By no means," says he, "does the Catholic Faith, which believes that Christ the Son of God was born of a virgin, according to the flesh, suppose that the same Son of God was so shut up in His Mother's womb, as to cease to be elsewhere, as though He no longer continued to govern heaven and earth, and as though He had withdrawn Himself from the Father. But you, Manicheans, being of a mind that admits of nought but material images, are utterly unable to grasp these things." For, as he again says (Ep. ad Volus. cxxxvii), "it belongs to the sense of man to form conceptions only through tangible bodies, none of which can be entire everywhere, because they must of necessity be diffused through their innumerable parts in various places... Far otherwise is the nature of the soul from that of the body: how much more the nature of God, the Creator of soul and body!... He is able to be entire everywhere, and to be contained in no place. He is able to come without moving from the place where He was; and to go without leaving the spot whence He came."
Reply to Objection 3. There is no uncleanness in the conception of man from a woman, as far as this is the work of God: wherefore it is written (Acts 10:15): "That which God hath cleansed do not thou call common," i.e. unclean. There is, however, a certain uncleanness therein, resulting from sin, as far as lustful desire accompanies conception by sexual union. But this was not the case with Christ, as shown above (Question 28, Article 1). But if there were any uncleanness therein, the Word of God would not have been sullied thereby, for He is utterly unchangeable. Wherefore Augustine says (Contra Quinque Haereses v): "God saith, the Creator of man: What is it that troubles thee in My Birth? I was not conceived by lustful desire. I made Myself a mother of whom to be born. If the sun's rays can dry up the filth in the drain, and yet not be defiled: much more can the Splendor of eternal light cleanse whatever It shines upon, but Itself cannot be sullied."
Source: Summa Theologica (New Advent)