How about some common sense.
Matt. 1:25 - this verse says Joseph knew her "not until ("heos", in Greek)" she bore a son. Some Protestants argue that this proves Joseph had relations with Mary after she bore a son. This is an erroneous reading of the text because "not until" does not mean "did not...until after." "Heos" references the past, never the future. Instead, "not until" she bore a son means "not up to the point that" she bore a son. This confirms that 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.
Not to go real heavy into the original Greek of that verse, but the actual Greek used for until is heos hou. Note that it can be read “before” or “until” but neither imply that “after Jesus was born”, that Mary and Joseph had sex. That’s not what the phrase reads…it reads that they did not have sex to insure that the readers and hearers of the Gospel would know there was no way that Jesus was Joseph’s biological son – St. Matthew put it there, through the inspiration of the Holy Spirit, to insure no misunderstandings. And it’s not just Catholics who “read it this way”. It is an ecumenical (multiple denominations) truth that the finest Biblical theologians and scholars – Catholic and Protestant – have attested to for decades.
Sometimes people fall into the trap that because their Bible occasionally refers to the “brothers of Jesus” (Mk 3:31-32, Jn 7:3-10) that Mary must have had other kids following Christ. Again, the translation is an important issue. First, realize that the Greek language is not like the English that you speak. The word “brothers” in these translations encompasses far more than “blood brothers” – it includes (but is not limited to)
stepbrothers,
adopted brothers,
cousins,
close family friends
and (in some cases) even the children of handmaids or servants.
Second, the “brothers of Jesus”, James and Joseph, are sons of another Mary whom St. Matthew un-affectionately called (the other Mary) (Mt. 28:1). Third, many Biblical scholars attest and believe that the “brothers” are sons of Joseph from a previous marriage – thus, making them stepbrothers. Though I’m not a Biblical scholar, after extensive, extensive study into the life of St. Joseph, I find this assertion to be quite a compelling argument, as well. Lastly, it was a commonly held belief that Mary was a perpetual virgin taught by the early reformers– which goes to show not the Church’s dominance in thought, but the commonly held Tradition passed down from generation to generation, especially among the Johnnine (St. John’s followers) communities of the early Church, with whom Mary lived following the Passion of our Lord (Jn 19:27).