Three Scriptural Arguments for Mary’s Perpetual Virginity

Here are a few of the basic passages that support Mary’s perpetual virginity:
1. Luke 1:34 - "Then Mary said to the angel, How shall this be, for I do not know man?"
There are two reasons this supports Mary perpetual virginity. First, the phrase “I do not know man” carries a sense of permanence, like “I do not smoke.”

Second, there is the context. Mary was betrothed to Joseph when the angel told her she would conceive a child. But her question shows that she knew she would not conceive a child with him. By asking how she was to conceive, she shows that she was not planning to. 
2. John 19:25-27 - "When Jesus therefore saw His mother, and the disciple whom He loved standing by, He said to His mother, Woman, this is your son! Then He said to the disciple, This is your mother!"
There are three reasons this supports Mary’s perpetual virginity. First, Jesus’ dying action was to do something to provide for Mary. That suggests that He had no brothers and sisters to take care of her.

Second, the fact that he chose St. John instead of a brother or sister suggests the same thing.

Third, in Greek the words Jesus uses contain a definite article: “This is the son of you,” not “this is a son of you,” suggesting that Jesus was Mary’s only son. 
3. Isaiah 7:14 - "Therefore the Lord Himself will give you a sign: Behold, the virgin shall conceive and bear a Son, and shall call His name Immanuel."
There are two reasons this supports Mary’s perpetual virginity. First, Mary’s virginity is supposed to be a “sign.” The reason God chose a woman who didn’t have children is because the presence of other children would obscure Jesus’ divine origin. That same reason would continue if the children were born either before or after Jesus.

Second, the passage says she will still be a virgin once she has given birth to Jesus. That can only happen by a miracle. But this supplies us with an argument for Mary's perpetual virginity: why would God have used a miracle to preserved Mary's virginity at the time of Christ's birth, if she was only to lose it soon after? This suggests she was not going to lose her virginity.