If the virginity of Mary before, during, and after the conception of her Divine Son was always considered part of the deposit of faith, this was done only on account of the historical facts and testimonials. The Incarnation of the Son of God did not in itself necessitate this exception from the laws of nature. Only reasons of expediency are given for it, chiefly, the end of the Incarnation. About to found a new generation of the children of God, The Redeemer does not arrive in the way of earthly generations: the power of the Holy Spirit enters the chaste womb of the Virgin, forming the humanity of Christ.
Many holy fathers (Sts. Jerome, Cyril, Ephrem, Augustine) say that the consent of Mary was essential to the redemption. It was the will of God, St. Thomas says (Summa III:30), that the redemption of mankind should depend upon the consent of the Virgin Mary.
This does not mean that God in His plans was bound by the will of a creature, and that man would not have been redeemed, if Mary had not consented. It only means that the consent of Mary was foreseen from all eternity, and therefore was received as essential into the design of God.

Holweck, F. (1907). The Annunciation. In The Catholic Encyclopedia. New York: Robert Appleton Company.

In the sixth month, the angel Gabriel was sent from God to a town of Galilee called Nazareth, to a virgin betrothed to a man named Joseph, of the house of David, and the virgin’s name was Mary.

And coming to her, he said, “Hail, favored one! The Lord is with you.”
But she was greatly troubled at what was said and pondered what sort of greeting this might be. Then the angel said to her, “Do not be afraid, Mary, for you have found favor with God. Behold, you will conceive in your womb and bear a son, and you shall name him Jesus. He will be great and will be called Son of the Most High, and the Lord God will give him the throne of David his father, and he will rule over the house of Jacob forever, and of his kingdom there will be no end.”

But Mary said to the angel, “How can this be, since I have no relations with a man?”

And the angel said to her in reply, “The holy Spirit will come upon you, and the power of the Most High will overshadow you. Therefore the child to be born will be called holy, the Son of God. And behold, Elizabeth, your relative, has also conceived a son in her old age, and this is the sixth month for her who was called barren; for nothing will be impossible for God.”

Mary said, “Behold, I am the handmaid of the Lord. May it be done to me according to your word.” Then the angel departed from her.

Luke 1:26-38