First, let’s go to the Catechism and see what it has to say:
The Hebrew used in Mark 3:31 doesn’t have the words differentiating “brother” and “cousin”. The same word would be used to describe “the closet relative”, not specifically brother or cousin.
The deepening of faith in the virginal motherhood led the Church to confess
Mary's real and perpetual virginity even in the act of giving birth to the Son
of God made man. In fact, Christ's birth "did not diminish his mother's
virginal integrity but sanctified it." And so the liturgy of the Church
celebrates Mary as Aeiparthenos, the "Ever-virgin."
Against this doctrine the objection is sometimes raised that the Bible mentions brothers and sisters of Jesus. The Church has always understood these passages as not referring to other children of the Virgin Mary. In fact James and Joseph, "brothers of Jesus," are the sons of another Mary, a disciple of Christ, whom St. Matthew significantly calls "the other Mary." They are close relations of Jesus, according to an Old Testament expression. (499, 500 CCC)
Now, look at this:
When Jesus saw his mother and the disciple there whom he loved, he said to hisIf Jesus had blood siblings from Mary (as argued above) then what Jesus does in John 19 would have been a breach of the law; something no faithful Jew (including Jesus) would do. When the oldest child dies, care of the parents goes to the next oldest. The Jews in Jesus’ time took their elderly parents into their homes to care for them. Jesus could never have given Mary away like this if he had siblings; it would have never been allowed.
mother, "Woman, behold, your son." Then he said to the disciple, "Behold,
your mother." And from that hour the disciple took her into his home. (Jn
So Mary didn’t have other children but we all know that it’s possible not to have children and not be a virgin, so why does the Church teach this about Mary?
It shouldn't come as a shock that the Catholic Church has a bit of a counter-culture view on sex. In fact, that may be the biggest understatement of all time. The Catholic view is that sex is a foreshadowing or a mirroring of the life we're given by God. A seed is given, it is received, and life begins. The Eucharist follows the same pattern, but more on that another day.
Actually, I think I'll use the Eucharist to illustrate where I'm going here. The Eucharist is a participation in both our Lord's suffering and death and in his glory in heaven. However, it's an earthly sacrament and isn't a full participation in the heavenly feast. When a person dies and goes to heaven, they won't still receive the Eucharist; there would be no point! Why receive the earthly sacrament when you can have the real deal in heaven?
Mary's virginity is the same way. If sex is a foreshadowing of God's love for us, why would she receive something less after receiving the perfect fulfillment of that love? Why participate in an imitation of the reality already experienced? For her, that relationship already exists, she carried God himself in her womb. What married couples immitate, she fulfilled.
Finally, Catholics believe Mary is the new Ark of the Covenant prefigured in the Old Testament. As such, she is the holiest of holies. She must be unblemished and pure. The Ark is where only God lives. No, we don't think Mary said "Sorry, Joseph, I'm the prefigured Ark so let's just watch TV." But we do believe that God afforded Mary and Joseph grace to keep her as she was prefigured to be; the Ark of the new Covenant.