Her whole earthly life? How can that be???

From the Catechism para. 491:

Through the centuries the Church has become ever more aware that Mary, "full of grace" through God (Luke 1:28) was redeemed from the moment of her conception. That is what the dogma of the Immaculate Conception confesses, as Pope Pius IX proclaimed in 1854: The most Blessed Virgin Mary was, from the first moment of her conception, by a singular grace and privilege of almighty God and by virtue of the merits of Jesus Christ, Saviour of the human race, preserved immune from all stain of original sin.

And para. 492:

The "splendour of an entirely unique holiness" by which Mary is "enriched from the first instant of her conception" comes wholly from Christ: she is "redeemed, in a more exalted fashion, by reason of the merits of her Son". The Father blessed Mary more than any other created person "in Christ with every spiritual blessing in the heavenly places" and chose her "in Christ before the foundation of the world, to be holy and blameless before him in love" (cf. Eph 1:3-4).

Why do we believe Mary stayed ever-virgin?