The Perpetual Virginity of Mary and Divine Faith
The Church's teaching on the Perpetual Virginity is from the patrimony of Sacred Tradition and must be acceded to by all Catholics.
In faith, contrary to reason alone, the surest argument is from Divine authority, i.e., simply because God reveals. God reveals through His Church, so the surest argument arises from the Church authoritatively asserting a truth from Divine Tradition, such as Mary being Perpetually Virgin. Catholics hold this dogma to be true because we know that the Church's Tradition authoritatively reveals God's mind on this.
Now one may believe certain religious doctrines that are primary: God, Trinity, Incarnation, Redemption, Bible, but on what basis? Is it because one has decided to accept these as plausible? like human faith? Or is it because one really believes God Himself, Who cannot be wrong or deceive and has given one the grace to believe these things, which are therefore Divine Mysteries, which our human minds cannot fathom? If the latter, one has Divine faith; if not, one has only human faith. But if the latter, that does not make one a Catholic ipso facto, because in order to have Divine and Catholic Faith one must accept that God is revealing in and through His Catholic Church infallibly. Otherwise one is a Protestant in one's divine belief.
Non-Catholics do not experience Revelation from God as necessarily mediated infallibly by the Church, and as a result do not accept as coming from God simply what the Church proposes as coming from God. They reserve the attitude of limiting by their own experience what the Church has a right to propose as from God; that is what Luther did, what every Protestant assumes is possible.
Some might respond that the Catholic Church cannot be wrong about the main things, but that it can be wrong about many other things that surround the main things (the main things again being Trinity, Incarnation, Redemption); but that means that like the Old Testament, God only sometimes reveals the truth to the People of God. It would not mean that whatever the Catholic Church proposes or believes as revealed is coming from God. It may just be a human institution being (like good parents) right on the main things and simply mistaken on a lot of things else.
But that reduces one's faith in the Church to only a human faith in an institution which is to be believed, even given the benefit of the doubt up to a point, or put on hold, until one's own experience convinces one that it is wrong on what it says is coming from God and you are right! In a word, private judgment. And voila, a Protestant masquerading as a Catholic, because born into the Church, but with the mental understanding of a Protestant.
The classical Catholic understanding is that human judgment makes it possible to see that Catholic faith could be true and is not unreasonable to accept. But that does not make one actually accept it. For that, Grace is needed, and access to that is by prayer and conversion of life, not by simple reasoning as an adult on experience.
As to the specific topic, the virginal birth of Christ points to a miraculous conception (that His Father by Nature is God, not man) by the Holy Spirit (indicating the Trinity of Divine Persons); that Mary had no more children is just one indication that Christ Jesus is different in some essential way from all other humans. If she had had other children there would be absolutely no reason to think Jesus was any different from those siblings in His origin, i.e., that He was God the Son become Man. Thus the Perpetual Virginity safeguards the specialness of Jesus as born miraculously, which safeguards His special conception, which manifests His Eternal Pre-Existence. So there IS a "logic" to the Incarnation of God as
The Man Jesus is claimed to be a Divine Person, always having existed eternally with His Father, Who descended to become a man. His conception must manifest this (He cannot have a human father). Thus the miraculous conception by the Holy Spirit overshadowing Mary and making from her flesh a Conceptus. Mary's virginity guarantees that He has no human father (
There is also the religious distinction between the sacred and the profane or ordinary. Something sacred is set apart from the ordinary only for Divine use or purpose. For example, one does not take the chalice used at Mass and use it for ordinary meals to drink soda from. Mary's womb is the sacred "chalice" containing the Divine Humanity of God the Son; her womb is to be used only for that Divine purpose -- because of the Holiness of God (God in the Judeo-Christian understanding is different from His Creation; and Holiness is the attribute of this difference). So there is a religious "logic" of the sacred as different from the profane, i.e., the creaturely element set apart only for Divine use or purpose, therefore never for ordinary use. It does not mean the ordinary is dirty or evil; the ordinary is good but not Divine. The sacred is taking something away from ordinary use and consecrating it for the Divine. That is why God would change the order of things for the unique Incarnation of His Divine Son.
Thus there are at least two sets of reasons for the Perpetual Virginity of Mary. The first is the one just mentioned: the Holiness of Mary's womb to be used for no other purpose but a thoroughly Divine one, never to be desecrated by ordinary use or purpose. Second, manifesting the Divine origin of her Child. If she had other children the ordinary way then why wouldn't her first child be just a human with a human father like His brothers and sisters? He must be a unique son of Mary to manifest being the unique Son of God the Father, having no biological father.
The Perpetual Virginity of Mary is in no way an implication that sex and marriage are anything less than what God designed them to be.(To make it clear, let me add that this is a sacred marital covenant which unity is expressed by the intimate relations between husband and wife.) Contrary to the usual either-or thinking found in Protestantism, Catholicism has always said that its both/and, and this is seen in the Church's teaching re the Virgin Mary.
In her person she mirrors the two truths of the sacredness of marriage and the goodness of virginity for the sake of the Kingdom. And
We cannot isolate The Incarnation of Christ from other factors (i.e., His being born a Jew; His being born of a particular woman; that particular woman being a virgin before His birth and remaining a virgin after; that particular woman remaining virginal in the midst of a marriage). Marital intimacy is not a desecration of any woman, nor is birth a desecration of any woman's womb, but it would be a desecration to impregnate in the ordinary way for ordinary purposes a womb consecrated to God. Especially in Mary's case God Himself consecrates her womb (as opposed to a human man consecrating a virgin to God). So the desecration violating what is holy would be unjust to God's honor, to say the least.
A greater familiarity with the Gospel of St. John can assist us in discovering the true Virginal Motherhood of Mary, particularly the scene at the foot of the Cross. There we have the Beloved Disciple, who is a type of all who will be followers of Christ, given to Mary, and John to her.
Her motherhood now encompasses all who are now brothers and sisters in Christ.