The word which is given here as "until" in the translation is "ἕως" in Greek. It does not make any implications about the state afterwards, sometimes it is also translated as "for".
Another verse using the same Greek word is Philippians 1:10: "so that you may be able to discern what is best and may be pure and blameless until the day of Christ". Does that mean that we should lose this ability after the day of Christ? Surely not.
Thus, it is not possible to make an argument against ever-virginity from Matthew 1.
But why do we affirm ever-virginity then? Because (oral) tradition teaches us, and this teaching was accepted by Orthodox Christians throughout history. (Actually, it was also affirmed by leading figures of the reformation, such as Luther, Zwngli, Calvin and John Wesley.)
While not directly taught in the Bible, Mary's ever-virginity is confirmed by John 19:26-27. If Mary had had more children, they could have taken care of her after Jesus' death. But apparently, there was no one around to do that, so Christ put Mary into the Beloved Disciple's charge.