"Until" in 17th c. English does not imply that the event in question ever happened. It was used more like we would use "before". In Numbers 12, Moses is said to have not spoken a falsehood "until the day of his death". That doesn't mean that he started lying on his deathbed! Similarly, Mary and Joseph were chaste "until" the birth of Jesus, thus showing that Christ's birth was miraculous. It doesn't imply that they then ceased living chastely.
I understand the explaination. I don’t however believe it. Yes it could have meant what you are saying. However, the Bible doesn’t state that she remained a virgin. Common sense would dictate that a man and woman living as husband and wife would have marital relations.
There are too many things to explain away and no direct confirmation that she remained a virgin. It wasn’t sinful for her to have relations with her husband so I am not sure why people have a problem believing that she did.