The cousin argument
In order for anyone to believe that Mary remained a virgin, they first have to believe the Bible doesn’t really say what it appears to say. The apostles called Jesus’ relatives brothers and sisters, but they really meant cousins? Then why didn’t they say cousins? Luke, a Greek speaking gentile, referred to Elizabeth as Mary’s cousin (syggenēs); why did he not call her Mary’s sister (adelphē)? The cousin exegesis makes Luke out to be inconsistent.
The Mary at the cross argument
They also propose the idea that Jesus had no brothers because on the cross He gave His mother to John. Someone has told you that it would have been customary for Jesus to place His mother’s care in the hands of His brother, and since He did not do that He must not have had a brother. Sounds logical, but there is one major problem; had Jesus given His mother to one of His brothers, His actions would have contradicted His teachings.
Jesus said, “My mother and my brothers are those who hear the word of God and do it.” I think you would agree that there is enough evidence from the Gospels to conclude that Mary believed in Jesus, but we know with certainty that His brothers did not (John 7:5). So if Jesus were to give His mother to His brothers He would have contradicted His own teaching.
What about the obvious conclusion just from reading the Bible?
I am sure most would agree that if a person who never heard the Gospel read them for the first time, they would believe that Mary had other children based simply on the text. It would take someone to come in and tell them that what they read isn’t as it appears. That means the burden of proof lies with those who dispute the obvious conclusion drawn from the text.
The evidence that the doctrine of Mary remaining in her virginity perpetually is not orthodox
“For the one and the same Spirit of God, who proclaimed by the prophets what and of what sort the advent of the Lord should be, did by these elders give a just interpretation of what had been truly prophesied; and He did Himself, by the apostles, announce that the fullness of the times of the adoption had arrived, that the kingdom of heaven had drawn near, and that He was dwelling within those that believe in Him who was born Emmanuel of the Virgin. To this effect they testify, saying, that before Joseph had come together with Mary, while she therefore remained in virginity, she was found with child of the Holy Ghost.” (Against Heresies; 3:4)
“And depreciating the whole of what appeared to be His nearest kindred, they said, Is not His mother called Mary? And His brethren, James and Joseph and Simon and Judas? And His sisters, are they not all with us? They thought, then, that He was the son of Joseph and Mary. But some say, basing it on a tradition in the Gospel according to Peter, as it is entitled, or The Book of James, that the brethren of Jesus were sons of Joseph by a former wife, whom he married before Mary. Now those who say so wish to preserve the honor of Mary in virginity to the end.” (Origen; Commentary on Matthew; 17)
“Thus is the temptation about His birth unsuitable, for it might have been contrived without any mention of either His mother or His brethren. It is clearly more credible that, being certain that He had both a mother and brothers, they tested His divinity rather than His nativity, whether, when within, He knew what was without; being tried by the untrue announcement of the presence of persons who were not present.” (Tertullian; On The Flesh of Christ; 7)
How and when did the doctrine take hold?
The belief that Mary remained a virgin seems to have developed out of the Christology debates of the mid to late fourth century. A very large schism occurred over the notion of whether or not Mary was the mother of God, i.e. Jesus’ divinity and humanity, or merely the mother of His’ humanity. The idea that Mary remained in her virginity blossomed out of that controversy. The only group I have come across from the ante-Nicene period that believed in Mary’s perpetual virginity is the Mary-worshiping Callyridians. And in my personal opinion, they are the catalyst for the Marian doctrines that developed in the fourth and fifth centuries.
So the evidence really is stacked against the belief that Mary remained a virgin. Historically it’s a no-brainer. But no amount of evidence will ever persuade people who just choose to believe the doctrine. But this evidence does matter to people who are willing to let go of ideology and seek truth.