“So the “perplexity” problem remains unanswered.”
To you it is an issue because, perhaps it is perceived to open the door to prebelief? In the end, there is no passage of Scripture at all that says Mary remained a virgin.
“I understand it seems obvious to you, which only raises the other question: why did nobody interpret i your way for 1500 years? “
This is a very big assumption, since you’ve yet to demonstrate it was believed before 100 ad, MDO.
More importantly, if something is falsely believed, regardless of how long, it remains false. In this case, no evidence it was taught by the Apostles before 100 ad. No declaration in the inspired Scriptures.
Your argument would be dispositive if we were arguing on the basis of Sola Scriptura; but we can't accept that principle, because it's historically untrue and logically impossible.
There was a NT Church for almost two decades before even the first book of NT Scripture was written (1 Thessalonians, around 51 AD at the earliest), and there were hundreds of Christian ecclesial communities comprising tens or hundreds of thousands of believers before the canon of the NT was closed (around 96 - 100 AD) --- and even then, most had never even seen a written copy.
The spread of the Holy Church far outstripped the writing, translation, copying and publication of Holy Text. The Church was based on Oral Tradition--- Apostolic preaching --- as the Written Tradition (Scripture) itself demonstrates.
The authority of Written Tradition itself is founded on the reliability and authority of the Oral Tradition which preceded it, sourced it, and provided the hermeneutic context for its correct interpretation.
Remember that even the canonization of the Written Tradition was based on the practice of the churches: the faith of the church was the exclusive source of the canon; the canon of Scripture was not the exclusive source of the faith of the church.
What are the principal truths we have from Oral Tradition? The basics of Sacraments, Liturgy, Apostolic Succession and hierarchy, the veneration of the Saints and of Mary, and the Canon of Scripture itself.
I think this does not interest you because it is not Sola Scripture. But as I said, Sola Scriptura is not tenable, and is not supported by the Bible itself.
In short, "Sola Scriptura" is un-Biblical. It's one of those troublesome "traditions of men". :o!
BTW, if you're ever in Rome, here's an inscription you can see for yourself in the 1st - 2nd century catacombs:
"Beata Maria Semper Virgine" (= "Blessed Mary Ever Virgin")