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
In order to believe in the immaculate conception you have to believe in the perpetual virginity. Scripture also states that Jesus has brothers and sisters. Jude being one of the half brothers.
Yes, the birth of Jesus is of Supernatural Origins, no doubt there. However, Mary was just like any other young Jewish girl, not of supernatural origins, especially if she was in the lineage of David, who had blood on his hands.
Until does mean up to a point when a change occurs, or Until wouldn't be there, it would state..." and knew her not for the rest of her life...".
Joseph and Mary were normal humans, they had to consumate the marriage, or there wouldn't be a "union"...
It's all rather simple really.
"Michal the daughter of Saul had no child until the day of her death."