The first thing to note about the perpetual virginity of Mary is that it’s the natural extension of the dogma of the virgin birth. Many modern people assume that, at its core, the virgin birth was basically a stunt. That is, the common modern assumption is that the meaning of Mary’s virginity is pretty much exhausted when somebody says, “Wow! She had a kid without the assistance of a man! Cool! He must be God Incarnate or something! Let’s check him out!”
The point is that the virginity of Mary is a sign, not a stunt. Stunts merely draw attention. They often don’t mean much beyond, “HEY!” And, at any rate, Jesus’ virgin birth drew no attention at the time it took place. But signs – and especially divine signs – are crammed with meaning. That is, signs signify. So the question becomes, “What did the virginity of Mary signify?” And the answer of the Catholic Church is that Mary’s perpetual virginity signifies crucial things, both about the “person of Christ and his redemptive mission” and “the welcome Mary gave that mission on behalf of all men” (CCC 499). And since, like all divine signs, this one goes on signifying long after its immediate time, Mary’s virginity is appropriate, fitting, and significant on a perpetual basis.