It doesn't. Just as it doesn't say that Mary was a PERPETUAL virgin.
Granted.
That's why there is no biblical basis for a Dogma that "Mary Had Sex" and nor is there any biblical basis for a Dogma of "Mary Had NO Sex EVER." Of course, the first doesn't exist. No denomination (known to me) has a formal doctrine of "Mary Had Sex." But two denominations (known to me) have a formal doctrine (in fact it's dogma) of "Mary Had NO Sex EVER."
You have an understanding of the nature of the Church that is fundamentally different from ours.
Do you agree that, by the Reformation (and for a while after), all known Christians were professing the ever-virginity of Mary? If not, kindly find some who weren't (as that would help satisfy the OP).
We're curious as to WHEN this idea that Mary WASN'T a Virgin first showed up
because to us the Church's teaching is something to be submitted to, not something that waits for us to prove.
I haven't read the entire thread, so forgive me if this is incorrect, but no Orthodox Christian would demand that you accept the Ever-Virginity of Mary unless you wanted to become Orthodox (since becomming Orthodox implies an acceptance of Orthodox Church teachings).
It is important PRECISELY because of its universality (the EO everywhere believes it), and its antiquity (there was never a time when we did NOT believe it). That's about as strong of a tradition as one can find, so when you (in another post) claim that scripture+tradition cannot substantiate the ever-virgin claim, you're incorrect. Our tradition most certainly DOES teach it, and as it isn't contradicted by Scripture (by your own admission), it would seem substantiated thereby.
As for its antiquity, that's what this thread is at least partly about. How old is the belief that she wasn't ever-virgin? Our liturgies (in quite old forms) contain that title for her... it was affirmed at (I think) the 5th ecumenical council, it may have been the 3rd (it was one of the anti-Nestorian councils).
So, this is a question of intellectual curiosity... what's the OLDEST record of someone proclaiming her NOT ever virgin?
Thus, the lack of substantiation is an "issue" for those two denominations with dogmas about her private sex life, not with those that don't.
Only if we accept the implied notion that we have to prove our positions to outsiders before they become legitimate for us to profess them.
And only if we DON'T accept the article in the Nicene Creed about having faith in the Church.
And only if we DON'T accept the traditional interpretation of calling the Church the pillar and ground of the Truth (on which that passage in the Nicene Creed is based).
Since you don't, its perfectly fine that you disagree with us. I would hardly pick this issue as a starting point for discussing Orthodoxy with someone. It's not a central teaching; but it IS a teaching, and barring some kind of evidence that it is NOT Holy Tradition (Apostolic), you're not likely to convince us that its ok to abandon it.
So the onus of proof is contingent on the goals of the people involved. We profess this as Truth, but do so by way of faith in the Church (not evidentialism, as you seem to be asking). That's not terribly persuasive to outsiders, but if our goal isn't to convince you of Mary's virginity
as an independent point then its ok that it isn't terribly persuasive.
If your goal is to convince us that it ought NOT be dogma (as seems to be your goal given your vehement emphasis on how problematic this dogma is), then the onus of proof would be on you, as it is you asserting that this is theologumenon (pious opinion) and not traditional teaching.
Show us the antiquity of the contrary opinion (that she was not ever virgin), and you'll have some ground to begin persuading us. If that isn't your goal, then that's ok. We can agree to disagree.
But that SHOULD then turn us to the OP's question: for curiosity's sake, how old IS the belief that Mary wasn't a virgin for her entire life?
In Christ,
Macarius