Tradition distinguishes the virginity of Our Lady before the birth, during the birth, and after the birth of Christ (ante partum, in partu et post partum). The present question concern the first aspect: did the Virgin conceive the Incarnate Word in a virginal manner?
Let us recall that continence is the fact of abstaining from acts of the flesh despite temptation. Chastity is the virtue which moderates the appetites according to reason. Perfect chastity consists of total abstinence. Virginity is the virtue that consist in perfect chastity kept forever. The word also designates corporal integrity.
Ante partum virginity (and post partum) has two aspects: one corporal, the conservation of the integrity of the flesh that the woman definitively loses through the marital act (virginal seal); and the other spiritual: the firm purpose not to use the marital act for the honor of God. Virginity in partu means the miracle of a birth also conserving the virginal seal intact.
Ante partum virginity, which is examined here, covers two things: Our Lady enjoyed this incorruption until the conception of Jesus, and the conception left it incorrupt.
Holy Scripture affirms the virginity of the Mother of God. In Isaiah: “Therefore the Lord himself shall give you a sign. Behold a virgin shall conceive, and bear a son,” (Is. 7:14). And again: “Now the generation of Christ was in this wise. When as his mother Mary was espoused to Joseph, before they came together, she was found with child, of the Holy Ghost,” (Mt. 1:18).
“And Mary said to the angel: How shall this be done, because I know not man? And the angel answering, said to her: The Holy Ghost shall come upon thee, and the power of the most High shall overshadow thee. And therefore also the Holy which shall be born of thee shall be called the Son of God,” (Lk. 1:34-35). Therefore, the Holy Virgin did not know man through her marriage, and she conceived Our Lord in a virginal manner.
The Lateran Council of 649 said explicitly: “If anyone does not properly and truly confess according to the holy Fathers, that the holy Mother of God and ever Virgin and immaculate Mary in the earliest of the ages conceived of the Holy Ghost without seed, namely, God the Word Himself specifically and truly, who was born of God the Father before all ages, and that she incorruptibly bore Him, her virginity remaining indestructible even after His birth, let him be condemned” Dz. 257.
It is defined de fide that the Mother of God is a virgin ante partum: the conception of Christ is virginal.
Theology shows the fitness of this definition by the Church
St. Thomas Aquinas gives several arguments for this fitness (III, 28, 1):
-- First, in order to reserve the paternity of the Christ to God the Father: therefore it was fitting that He have no father after the flesh.
-- The subject of this conception is the Word, begotten from all eternity through the most pure act of knowledge of God the Father; it is thus fitting that His generation according to human nature imitate the purity of His eternal generation. This argument is equally valid of virginity in partu.
--It was fitting that he who came to give spiritual birth through divine virtue (by the grace of God), should be born without fleshly commerce by divine virtue.
--The immediate principle of natural human conception is the will of man. Now the Incarnation transcends every human principle. The virginal conception manifests more clearly that this is an exceptional divine work.
In this light, the virginity of the Mother of God is a magnificent image of God’s holiness.