Thank you for your question. Actually two very good questions: One about Tradition and the other about the Perpetual Virginity of Mary.
First, let's deal with the different kinds of traditions. Indeed Jesus Himself warned against the traditions of men that made void the Word of God.
But the word tradition simply means teachings or something handed down. Therefore God's Word is Tradition . . . it has been preserved and handed down. Moreover, it was compiled into one collection of books, based on the Traditions handed down by the Apostles to their successors, who in turn handed it down to their successors, and so on.
While all of Scripture was written by the end of the first century, exactly what was and wasn't Scripture wasn't decided until 382 A.D. at the Council of Rome. A decade or so later the Councils of Hippo and Carthage, affirmed the same 73-book canon but the first, Church-wide Ecumenical Council to affirm the Canon was the Second Council of Nicaea in 787 A.D, the last Church wide Council before the Church split into East and West.
There is no Scripture text within the Bible that gives us a list of books is there?
But there are Scriptures that talk about the authority of the Church and importance of Godly Tradition.
St. Paul tells Timothy the "pillar and foundation of the truth is the Church." (1 Timothy 3:15)
St Paul also writes to the Thessalonians:
"Now we command you, brethren, in the name of our Lord Jesus Christ, that you withdraw yourselves from every brother that walks disorderly, and not after the tradition which he received from us."
(2 Thessalonians 3:6)
Therefore, brethren, stand fast and hold the traditions which you have been taught, whether by word, or our epistle.
(2 Thessalonians 2:15)
One should particularly take note of this verse, because Paul, writing under the inspiration of the Holy Spirit, tells us that God's Word or Instructions comes to us either in writing or by oral tradition.
So all traditions don't make void the Word of God, in fact without Tradition, we would not have the [Written] Word of God.
OK, Let's now talk about what Catholics consider Tradition with an uppercase T and traditions with a lower case t.
The uppercase Tradition is also called Sacred Tradition, Apostolic Tradition, Apostolic Teaching, etc. This is what we hold to as a matter of faith [and/or] morals handed down in some form from the beginning.
Then there are traditions or disciplines which are changeable things we do to express our faith or worship. They vary from what language we celebrate the Mass in to days we might fast. These aren't doctrinal matters; they exist for pastoral reasons and are often temporary.
Now to Mary. Some of the earliest Church's writings indicate that Mary remained a virgin after the birth of Jesus. Indeed it is implicit in the text of the Annunciation if you look at it carefully and take off your Protestant glasses.
When Gabriel announced that Mary was to conceive, he puts it in the future tense. So she had no reason to ask how will this be. She was engaged or betrothed to Joseph. Mary was old enough to know the biology. She rightly would have assumed that she and Joseph would have child in a normal way. Yet she pauses and says how is this to be. I do not know man. Well at that point she didn't but, as I pointed out, she was about to relatively soon. If she intended to have a normal marriage, then the question makes no sense.
We know from history, that certain women were dedicated to the Temple to remain virgins. They were given old men to be husband protectors who would also support them but the women remained consecrated virgins.
Our Tradition is that Mary was such a woman and it is the only way her question to Gabriel makes a whole lot of sense.
Now there are some other texts you might want to point out to dispute our claim so let me address them.
The text says that Mary did not know Joseph until after Jesus was born. OK, in English, it does seem to imply that after Jesus was born, Joseph and Mary had a normal marriage however the Greek word used in this text for until doesn't necessarily have such a meaning. It's the same word used in the Greek translation of the Old Testament in Samuel when David's wife Michal mocks him. That text says she didn't have any more children until she died. (2 Samuel 6:20-23)
- So does that mean she had children after she died?
The Bible also says Jesus shall reign until He has made His enemies His footstool.
(1 Corinthians 15:20-26)
- So does that mean He will stop reigning once His enemies are underfoot?
Now let's deal with the so-called brothers and sisters of Jesus, mentioned in the Gospels.
Well, we need to understand that culture. Lot was Abraham's nephew, yet at one point he is referred to as Abraham's brother. The term brother was often used to mean any relative so it's possible they were simply cousins. Or they could have been sons and daughters of Joseph from a prior marriage. Joseph was significantly older than Mary, and probably widowed.
- Does this definitively prove Mary's perpetual virginity from Scripture?
No, it does not but it certainly proves that doctrine doesn't oppose Scripture as you claim.
So it is certainly not a tradition that makes void God's Word which, as I've discussed, is a Tradition, to begin with.
We also know that all Christians follow the Tradition of monogamy even though it's not expressly taught in Scripture. In the Old Testament all the Patriarchs had multiple wives and in the Psalms David's many wives were called blessings from God so while we can read monogamy implicitly in texts like Genesis 2 and Ephesians 5, there is no clear cut text that says men can only have one wife.
In fact when Paul tells Timothy and Titus how to pick priests and bishops, he says pick men that only have one wife. From this, one could interpret that there were early believers from the Jewish tradition and other pagan traditions that had more than one wife and that they shouldn't be picked to be elders and overseers, meaning priests and bishops.
Yet all Christians today accept that polygamy is a sin. That is a Tradition but it's not explicit in the Bible.
Do you want to argue that this Tradition makes void the Word of God?
Warmly, Under His Mercy,