I have always been puzzled why the members of the church of Christ are so adamant that Mary, the mother of Jesus, had additional children with her husband, Joseph. I know the Catholic teaching is that Mary was a perpetual virgin; why does it matter to the church of Christ if she had other children or not? Jesus is the only one who was significant.
"All Scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness, that the man of God may be complete, thoroughly equipped for every good work" (II Timothy 3:16-17).
"Now we have received, not the spirit of the world, but the Spirit who is from God, that we might know the things that have been freely given to us by God. These things we also speak, not in words which man's wisdom teaches but which the Holy Spirit teaches, comparing spiritual things with spiritual" (I Corinthians 2:12-13).
Since the Bible is from God and the very words it contains were selected by God, it follows that accurately presenting what God taught in the Bible is important. Paul showed that small, seemingly insignificant details can have important meaning. For example, the lack of a plural changes a promise from a nation being a blessing to the world to a person (Christ) being a blessing to the world. "Now to Abraham and his Seed were the promises made. He does not say, "And to seeds," as of many, but as of one, "And to your Seed," who is Christ" (Galatians 3:16). The Jews of Paul's day had missed that small, but important point. Jesus used a very odd verse to prove that there was life after death: "But concerning the resurrection of the dead, have you not read what was spoken to you by God, saying, 'I am the God of Abraham, the God of Isaac, and the God of Jacob'? God is not the God of the dead, but of the living" (Matthew 22:32-33). It quickly is apparent that every word is important in the Bible, even when we might not fully appreciate what the significance is at the moment.
The Gospels are quite clear that Mary was not a virgin after she gave birth Jesus. "Then Joseph, being aroused from sleep, did as the angel of the Lord commanded him and took to him his wife, and did not know her till she had brought forth her firstborn Son. And he called His name Jesus" (Matthew 1:24-25).
Further evidence is found in the fact that Jesus was heir to the throne of David. This inheritance came through him through Joseph because Joseph married Mary prior to Jesus' birth. Jesus, being the first born received the blessing (Matthew 1). This becomes important because one way to explain away Jesus' siblings is to claim that they were Joseph's children by a previous wife. "Is this not the carpenter's son? Is not His mother called Mary? And His brothers James, Joses, Simon, and Judas? And His sisters, are they not all with us? Where then did this Man get all these things?" (Matthew 13:55-56). If Joseph had four sons older than Jesus, then Jesus would not be the heir to David's throne because the brothers outlived Jesus and would be in line to the throne prior to Jesus. Therefore the four sons had to have been born after Jesus.
Another tactic is to claim that "brother" in Greek can refer to a cousin, such is true. But what is ignored is that "sister" in Greek is not used to refer to a cousin. Jesus had both brothers and sisters.
Finally, Mary is never called a virgin after Jesus was born in the Bible.
Since the evidence in the Scriptures is solid, then the question of why Christians say that Mary did not remain a virgin after giving birth to Jesus is self-evident. We do so because it is the truth.
A better question, however, is to ask why the Roman Catholic Church established a tradition contrary what is found in the Bible. What is it that they seek to gain?
Roman Catholicism sees the celibate life to be superior to a sexual life. It is one of the reason why they forbid their priests, monks, and nuns from marrying. They hold this position despite Paul's warning: "Now the Spirit expressly says that in latter times some will depart from the faith, giving heed to deceiving spirits and doctrines of demons, speaking lies in hypocrisy, having their own conscience seared with a hot iron, forbidding to marry, and commanding to abstain from foods which God created to be received with thanksgiving by those who believe and know the truth" (I Timothy 4:1-3). Roman Catholics wish to elevate Mary above other humans and, thus, the perpetual virginity of Mary is a part of their claim that Mary was superior to others. The thought that Mary would have sexual relations with a man after having conceived a child by the Holy Spirit is abhorrent to a culture that seeks to make Mary someone more that the Scriptures teach. See: The Glorification of Mary
Therein lies the chief problem. Mary never held a special position beyond the privilege of having been the one to bear the Son of God and raise him. There is one mention of Mary after Jesus' death (Acts 1:14) and no mention of her after the church began. She had no special position in the church. All that the Roman Catholics attribute to her is of their own imagination -- unsupported by God's Word.
So why is the knowledge that Mary had other children important? Simply because it is evidence that the Roman Catholic Church teaches falsehoods.