That Christ was born of virgin without prejudice to His true and natural Humanity

GOD'S power being infinite, and all other causes deriving their efficacy from that, any effect produced by any cause may be produced by God without aid of that cause, and yet be of the same species and nature as though it had been produced in the ordinary way. As then the natural power of the human semen produces a true man, having the species and nature of a man, so the divine power, which has given that power to the semen, may produce the effect of that power, without calling the cause into activity, and so constitute a true man, having the species and nature of a man. Nor is anything lost to the dignity of the Mother of Christ by the virgin conception and birth: there is nothing in that to prevent her being called the Mother of the Son of God: for by the working of divine power she supplied the matter physically requisite for the generation of the body of Christ: which is all that a mother need do.

4.44 : That the Human Nature, assumed by the Word, was perfect in Soul and Body in the instant of Conception
4.46, 47 : That Christ was conceived by the Holy Ghost