The real question is why, from the Patristic era on, was trying to show “perpetual virginity” so important?
If sex within marriage is holy and a good thing, then why should it prove Mary as somehow more holy if she refused to have normal relations with her husband (just like St. Paul commanded of all married couples) after Jesus’ birth?
The answer is easy: Early pagan prejudices creeping into the Church, which made otherworldly chastity seem more holy and righteous. Gnostic influences were strong (body/material is bad....soul/spiritual is good...) and since sex, and specifically women, were associated with the “prison” of the material body, naturally, the most holy woman of all time, MUST have refrained from dirty, nasty, sex...FOREVER!
Of course the Hebrew scriptures (the Old Testament) know nothing of this, and neither did the Apostles who wrote the New Testament, but hey, if later bishops of Rome and other sages said it, it has to be true, right?
I think you’re absolutely correct.
Though those without ears to hear or eyes to see will never admit it short of a Damascus road level miracle.
†Paul said it is a debt owed to the spouse.
The protestant obsession with Mary's sex life is creepiness in action.
Speaking the Truth is important to the Catholic Church. It is what She does.