Mary was a virgin when she bore Jesus. Here are other texts that prove "not until" means "not up to the point that":
Matt. 28:29 - I am with you "until the end of the world." This does not mean Jesus is not with us after the end of the world.
Luke 1:80 - John was in the desert "up to the point of his manifestation to Israel." Not John "was in the desert until after" his manifestation.
Luke 2:37 - Anna was a widow "up to the point that" she was eighty-four years old. She was not a widow after eighty-four years old.
Luke 20:43 - Jesus says, "take your seat at my hand until I have made your enemies your footstool." Jesus is not going to require the apostles to sit at His left hand after their enemies are their footstool.
1 Tim. 4:13 - "up to the point that I come," attend to teaching and preaching. It does not mean do nothing "until after" I come.
Gen. 8:7 - the raven flew back and forth "up to the point that" [until] the waters dried from the earth. The raven did not start flying after the waters dried.
Gen. 28:15 - the Lord won't leave Jacob "up to the point that" he does His promise. This does not mean the Lord will leave Jacob afterward.
Deut. 34:6 - but "up to the point of today" no one knows Moses' burial place. This does not mean that "they did not know place until today."
2 Sam. 6:23 - Saul's daughter Micah was childless "up to the point" [until] her death. She was not with child after her death.
1 Macc. 5:54 - not one was slain "up to the point that" they returned in peace. They were not slain after they returned in peace.