Virginity is hardly in fashion these days. It is undervalued and some go as far as to challenge Mary's virginity.
Before Mary's time—although the Jewish people considered a woman's sterility as something shameful—elements leading to the idea of Christian virginity were underway. For instance, great importance was given to the virginity of the betrothed. We see it also in the context of God's promises and the Covenant. However, with Mary virginity takes on a new importance. Mary is the only woman of the New Testament to receive the title of virgin (Lk 1: 27; Mt 1: 23).
We need to examine two facts which are interdependent but appear to be separate: the virginal conception (Lk 1: 35) and Mary's perpetual virginity (Lk 1: 34).
First of all, we must understand the difference between the Immaculate Conception and virginal conception. The latter refers to the fact that Mary was conceived without original sin, the former that Mary conceived Jesus of the Holy Spirit while remaining a virgin. On a philosophical level, an atheist or an agnostic could logically affirm that virginal conception is impossible. Not so for those who believe God exists. As a matter of fact, if we admit the existence of a God who created all things, in the name of what principle can we deny Him the possibility of making a virgin conceive?
If, on the other hand, the story of the virginal conception is but a myth or a legend, what of the Incarnation, of the Redemption of the world through the Passion and Death of Christ, of the Resurrection and of the Parousia? It is striking to see that those, in the post-biblical era, who were denying the virginal conception were also denying the Divinity of Jesus. Thus Christianity as a whole was undermined.
Concerning Jesus' brothers:
From the first centuries, the Church's tradition upheld the virginal conception, as Saint Ignatius of Antioch testifies. Concerning Mary's perpetual virginity, there is the question of the Gospel's episodes which speak of Jesus' brothers. While in Greek two words are used to designate "brother" (adelphos) and "cousin" (aneptios), Aramean and Hebrew use one word. These languages designate close family ties by the words "brothers" and "sisters." Moreover, there is a passage in John's Gospel which strongly points to the fact that Jesus was Mary's only child (Jn 19: 25-27). Mary is alone with her Son at the foot of the Cross and Jesus entrusts her to the apostle John. If Mary had had other children, wouldn't Jesus normally have entrusted His mother to them? Much more could be said about Mary's virginity and the profound beauty of this poignant reality.