That does not say anything about Joseph having sex with Mary. You are confusing the English word "until" with the Aramaic and Greek definition of the transliteral. To them, it doesn't say anything about future, only about what it describes up to the present. You are presuming that the Bible says that Joseph had sex with Mary AFTER Jesus was born, but it DOES NOT SAY THAT. That is YOUR presumption.
Regards
Every time the phrase is used in the Bible it is talking about intercourse.
Genesis 38:26 And Judah acknowledged them, and said, She hath been more righteous than I; because that I gave her not to Shelah my son. And he knew her again no more.
Judges 19:25 But the men would not hearken to him: so the man took his concubine, and brought her forth unto them; and they knew her, and abused her all the night until the morning: and when the day began to spring, they let her go.
1 Kings 1:4 And the damsel was very fair, and cherished the king, and ministered to him: but the king knew her not.
Matthew 1:25 And knew her not till she had brought forth her firstborn son: and he called his name JESUS.
Let's see whose presumptions are more reasonable sounding. Assume there is a verse in the Bible that says "... and the hungry alligator did not eat the chicken unto he had rushed upon it". Now we have the question "Does the Bible say that an alligator ever ate a chicken?" Full Court says "YES!" But you, OTOH, would say "No, no, no. I have extra-Biblical Tradition which clearly says that when the alligator got there, he and the chicken had a tea party and then had a pillow fight. Therefore, the Bible must be interpreted in this light. The Bible never says anything like what you PRESUME".
Clearly, if we even grant you your point on the "until" "unto" issue (which I do not think we are obligated to do), both sides makes presumptions about the future. I am confident that logic, and human experience strongly favor our presumptions and strongly discredit your Tradition.