OK, and thanks for the explanation. I suppose my sneaky little mind was wondering if this belief was somehow tied to another. :) IOW, is some other (really important) belief logically impossible UNLESS Mary remains a virgin. The obvious one is that Jesus could not then have blood siblings. But even here, I don't see how this is a world-ender in terms of importance. I can't figure how either of our respective core principles is strongly affected based on what the truth of this is, but I could be wrong. It's still fun to debate anyway. :)
Since you believe that the only evidence that is valid from the 1st c. is what is found in the New Testament, and reject all histories other than the NT (at least all other Christian histories), ...
For myself, I wouldn't go so far as to say I throw out all histories except the NT. I remember one time I was teaching from John, and I made a big deal about the scene of Jesus crossing the Kidron Valley on His way to the garden to be arrested. After researching, I noted that the creek running through the Valley led up to the Temple where all of the sacrifices were being made.
It's a simple fact that with all the thousands of sacrifices, that there would be a lot of blood, and that blood had to go somewhere. I said that this blood was emptied into the creek. So, my point was to note the irony of Jesus literally stepping over a creek carrying sacrificial blood, as He himself went to become a sacrifice. All of this, of course, is extra-scriptural, but it was so beautiful, I couldn't resist sharing it. :)
I'm not sure what other belief Mary's ever-virginity would be tied to in your sneaky mind. I can't think of why her not remaining a virgin after her birth would affect our salvation. We do believe that Joseph, after all of the visions and angels and shepherds and virgin birth and so forth realized that this child was the Messiah and that this Messiah was perhaps more than they had ever suspected he would be. Whether he fully realized from the first that Jesus was God, we of course do not know. But in light of this, we find it unlikely that he would treat Mary as an "ordinary wife."
This is of course a retrospective comment, and it is *not* the reason why we believe in the ever-virginity of the Theotokos. There is not a "she must have been ever-virgin in order that ____ could be true" involved at all.
"I can't figure how either of our respective core principles is strongly affected based on what the truth of this is, but I could be wrong."
I don't either. Which is why I am frankly surprised at the level of hostility to the belief in her ever-virginity amongst the Protestants on this forum. For her to be ever-virgin affects no Protestant doctrine whatsoever, and I don't think it affects our core doctrines, either. For that reason, I do not feel that I am being untrue to my own beliefs by saying that I can understand why Protestants would believe as they do -- and yet there is a rigid unwillingness on the part of Protestants on this thread to acknowledge that *just maybe* it is reasonable for us to believe as we do.
With regard to your story from your teaching, I'm afraid that you made my point. Protestants will mention interesting things from archeology, Jewish histories or traditions, and secular histories as being plausible and worth mentioning. But anything that is written by the Church of the early centuries is suspect at best, verboten at worst. There is a double standard at work, where the presumption is that what the Church believed in the early centuries (anything but the New Testament as read through Protestant eyes) is untrue unless somehow conclusively proved otherwise, whereas other archeological and historical accounts or theories are accepted with relative ease.
I just find it interesting.