The question could be inverted. Why not believe in her ever-virginity? The Eastern Church has witnessed to the perpetual virginity of the Theotokos steadfastly for two thousand years and shows no sign of tiring. In the West, the idea was largely undisputed until late in the Reformation; even Luther and Calvin accepted the tradition.
Indeed, to suggest (a) that the tradition about her perpetual virginity could have been introduced after apostolic times, (b) that this tradition would have gone little noticed by a Church in the throes of questioning everything about what it believed in the first millennium, (c) that such a novel tradition should be considered inconsequential enough to pass without discussion before it became universally proclaimed, and (d) that such a tradition should have no discernible literary or geographical origin and yet be universally accepted from very early in the Church’s history, is to form a very unlikely hypothesis
Set Apart to God
To argue against Mary’s perpetual virginity is to suggest something else that is greatly implausible, not to say unthinkable: that neither Mary nor her protector, Joseph, would have deemed it inappropriate to have sexual relations after the birth of God in the flesh. Leaving aside for a moment the complete uniqueness of the Incarnation of the Second Person of the Trinity, recall that it was the practice for devout Jews in the ancient world to refrain from sexual activity following any great manifestation of the Holy Spirit