The Catholic Church teaches that Mary was a virgin for her whole life. Naysayers insist that the word "until" means that she was not a virgin after the birth of Jesus, but the Bible doesn't support this conclusion.
(Matthew 1:25) And knew her not till she had brought forth her firstborn son: and he called his name JESUS.
Certainly there are examples of the use of the word "until" in which there is a change in condition afterwards:
(Matthew 2:15) And was there until the death of Herod: that it might be fulfilled which was spoken of the Lord by the prophet, saying, Out of Egypt have I called my son.
(Matthew 17:9) And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead.
(Luke 1:20) And, behold, thou shalt be dumb, and not able to speak, until the day that these things shall be performed, because thou believest not my words, which shall be fulfilled in their season.
However, there are also plenty of examples in which there is no change in condition whatsoever after the word "until":
(Luke 24:49) And, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem, until ye be endued with power from on high.
They remained (tarried) in Jerusalem long after this.
(John 9:18) But the Jews did not believe concerning him, that he had been blind, and received his sight, until they called the parents of him that had received his sight.
It turns out that they didn't believe them after this either.
(Matthew 12:20) A bruised reed shall he not break, and smoking flax shall he not quench, till he send forth judgment unto victory.
Jesus never discourages the poor and weak, he is always merciful.
(Acts 23:1) And Paul, earnestly beholding the council, said, Men and brethren, I have lived in all good conscience before God until this day.
Paul doesn't stop living a holy life after this.
(Romans 5:13) (For until the law sin was in the world: but sin is not imputed when there is no law.
Sin is still in the world even after the coming of Christ.
(Romans 8:22) For we know that the whole creation groaneth and travaileth in pain together until now.
This condition persists even today — it didn't end when this letter was written.
(Philippians 1:5) For your fellowship in the gospel from the first day until now.
Their fellowship didn't end when Paul wrote this letter.
(1 Timothy 4:13) Till I come, give attendance to reading, to exhortation, to doctrine.
They are to do these things even after Paul comes.
Therefore, the presence of the word "until" simply cannot be used to prove that Mary was not a virgin for her whole life.