Roman Catholics, Eastern Orthodox, and Oriental Orthodox teach in the doctrine of Perpetual Virginity of Mary that Mary remained a virgin all her life. The Bible does not support this idea.
No where does the Bible state that Jesus was the only son of Mary, but it DOES say Jesus was GOD’s only Son in John 3:16:
16 For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.
Notice that Scripture says Joseph had no sex with Mary until she gave birth to Jesus. This says Joseph and Mary had no sex up to the point of when Jesus was born. She was a virgin when Jesus was born, but the word ‘until’ implies afterwards Joseph had sex with her.
When Joseph woke up, he did what the angel of the Lord had commanded him and took Mary home as his wife. But he had no union with her UNTIL she gave birth to a son. And he gave him the name Jesus.
Also, she had other children later – at least James, Joseph, Judas, and Simon, as well as at least two sisters.
55 “Isn’t this the carpenter’s son? Isn’t his mother’s name Mary, and aren’t his brothers James, Joseph, Simon and Judas? 56 Aren’t all his sisters with us? Where then did this man get all these things?
“Where did this man get these things?” they asked. “What’s this wisdom that has been given him, that he even does miracles! Isn’t this the carpenter? Isn’t this Mary’s son and the brother of James, Joseph, Judas and Simon? Aren’t his sisters here with us?” And they took offense at him.
The word translated as ‘brother’ is adelphos, and ‘sister’ is adelphe. Although these words can mean cousins or kinsmen, the actual meanings of the words indicate brothers and sisters of the same mother. It is interesting to note that every place where adelphos is used in the Gospels in relation to a named person, it speaks of people as actual brothers.
If they were cousins, why would they show up with her so often?
31 Then Jesus’ mother and brothers arrived. Standing outside, they sent someone in to call him. 32 A crowd was sitting around him, and they told him, “Your mother and brothers are outside looking for you.”
19 Now Jesus’ mother and brothers came to see him, but they were not able to get near him because of the crowd. 20 Someone told him, “Your mother and brothers are standing outside, wanting to see you.”
12 After this he went down to Capernaum with his mother and brothers and his disciples. There they stayed for a few days.
They all joined together constantly in prayer, along with the women and Mary the mother of Jesus, and with his brothers.
1 Corinthians 9:5
5 Do we not have a right to take along a believing wife, even as the rest of the apostles and the brothers of the Lord and Cephas?
Also, when one uses the term ‘firstborn’, this implies there are other children born afterwards.
6 While they were there, the time came for the baby to be born, 7 and she gave birth to her firstborn, a son. She wrapped him in cloths and placed him in a manger, because there was no room for them in the inn.
Then we have the extra-biblical evidence. Josephus wrote in “Antiquities of the Jews Book XX”, “Festus was now dead, and Albinus was but upon the road; so he assembled the sanhedrim of judges, and brought before them the brother of Jesus, who was called Christ, whose name was James, and some others, [or, some of his companions]; and when he had formed an accusation against them as breakers of the law, he delivered them to be stoned” (http://www.ccel.org/j/josephus/works/ant-20.htm). Now as to whether Josephus was lying or not, we have the archeological find of the James Ossuary. An ossuary is a box used to house the bones of someone after the flesh, muscles, and organs had decayed. It was dated to 1st century AD, before 70AD. This particular ossuary had the inscription, “James the son of Joseph the brother of Jesus.” In all fairness, it is slightly possible this is not of James the Just, brother of Jesus of Nazareth, but all signs point to it indeed being the bone box of the brother of Jesus Christ (http://biblicalstudies.info/James/james.htm). Conservative estimates say there is the possibility of up to 3 people in Jerusalem at the time who matched the description of “”. Others say there is no question this box held the bones of the brother of Jesus of Nazareth (http://www.biblenetworknews.com/northamerica/112302_canada.html).
Mary was a virgin when she became pregnant with Jesus and when she delivered Him, but she was not one afterwards; she had sex and gave birth to other children. To believe otherwise is to add to the Bible and change the meaning of words that are in the Bible showing otherwise.