Knew her not = Old english means, didn't have sex. The kicker... the word until she had born a son...
So after Jesus was born, Joseph and Mary were just like any Jewish couple
Nice, but you are reading more into that than what is intended. It is a testament that the birth was of supernatural origin.
To read it the way you are is to make it say they had relations immediately after she gave birth.
Sometimes "until" is used in the Bible and elsewhere to simply mean "up to that point in time." It doesn't mean things change afterwards.
SD
Okay, so usign the same reasoning concerning your grasp of the English language:
"Therefore Michol the daughter of Saul had no child till the day of her death." (2 Samuel 6.23)
So Michol had children after she was born? That is the same reading you are putting on St. Matthew 1.25
"And he buried him in the valley of the land of Moab over against Phogor: and no man hath known of his sepulchre until this present day." (Deuteronomy 34.6)
So we now know the burial place of Moses?
"But Philip was found in Azotus: and passing through, he preached the gospel to all the cities, till he came to Caesarea." (Acts 8.40)
So did St. Philip stop preaching once he reached Caesarea?
Maybe your problem is that you don't know the definition of the word until. Until means "up to the time of/that". It doesn't speak to what happens in the future.