Thomas Aquinas, Compendium of Theology

Since, as we have shown, the Son of God was to take flesh from matter supplied by human nature, and since in human generation the woman provides matter, Christ appropriately took flesh from a woman. This is taught by the Apostle in Galatians 4:4: "God sent His Son, made of a woman." A woman needs the cooperation of a man in order that the matter she supplies may be fashioned into a human body. But the formation of Christ's body ought not to have been effected through the power of the male seed, as we said above. Hence that woman from whom the Son of God assumed flesh conceived without the admixture of male seed. Now the more anyone is detached from the things of the flesh, the more such a person is filled with spiritual gifts. For man is raised up by spiritual goods, whereas he is dragged down by carnal attractions. Accordingly, since the formation of Christ's body was to be accomplished by the Holy Spirit, it behooved that woman from whom Christ took His body to be filled to repletion with spiritual gifts, so that not only her soul would be endowed with virtues by the Holy Spirit, but also her womb would be made fruitful with divine offspring. Therefore her soul had to be free from sin, and her body had to be far removed from every taint of carnal concupiscence. And so she had no association with a man at the conception of Christ; nor did she ever have such experience, either before or after.

This was also due to Him who was born of her. The Son of God assumed flesh and came into the world for the purpose of raising us to the state of resurrection, in which men "shall neither marry nor be married, but shall be as the angels of God in heaven" (Matt. 22:30). This is why He inculcated the doctrine of continence and of virginal integrity, that an image of the glory that is to come might, in some degree, shine forth in the lives of the faithful. Consequently He did well to extol purity of life at His very birth, by being born of a virgin; and so the Apostles' Creed says that He was "born of the Virgin Mary." In the Creed of the Fathers He is said to have been made flesh of the Virgin Mary. This excludes the error of Valentinus and others, who taught that the body of Christ was either phantastic or was of another nature and was not taken and formed from the body of the Virgin.