Was Mary a perpetual virgin?

Catholic doctrine and many church fathers believe in the perpetual virginity of Mary. That Mary was a virgin when she conceived Jesus is explicitly documented in the Bible and is universally believed by Christians. The doctrine of perpetual virginity is early but more contentious. It is defended by appealing to its wide support in the early church and for many years, even believed by some protestants such as Luther, Zwingli, and Wesley.

It is claimed that Mary took a vow of perpetual virginity and that a guardian was appointed her, an old widower who would marry her but not consummate the marriage. Thus Joseph was betrothed to her then married her but never consummated the marriage. Jesus' brothers were either his half brothers through Joseph, or Jesus' cousins.

Biblically the defence is much weaker. The relevant verses are

but [Joseph] knew her not until (ἕως) she had given birth to a son. And he called his name Jesus. (Matthew 1:25)

And Mary said to the angel, “How will this be, since I am a virgin?” (Luke 1:34)

When Jesus saw his mother and the disciple whom he loved standing nearby, he said to his mother, “Woman, behold, your son!” Then he said to the disciple, “Behold, your mother!” And from that hour the disciple took her to his own home. (John 19:26-27)

Matthew is interpreted to mean Joseph did not know her before the birth and thenceforth. Luke is read as a vow of perpetual virginity. John is read as implying Mary had no other sons therefore Jesus handed his responsibility for his mother to John.

I hold tradition in some regard but hold Scripture higher. I think the verses espoused as proof of Mary's ongoing virginity as weak, and other verses suggest otherwise. Dealing with the above verses first.

Matthew 1:25 is usually translated as until. Most versions do this. Some argue that a translation such as

but [Joseph] knew her not before she had given birth to a son. And he called his name Jesus. (Matthew 1:25)

is allowed. "Until" implying that Joseph abstained from sex prior to but not after the birth, whereas "before" only makes claims of abstinence up until to the birth and doesn't imply anything about their conjugal activity after the birth. The problem with this is even if we accept a translation using a more neutral preposition, the context implies sexual activity after the birth. Coitus is so connected to marriage that it is assumed without any information. For coitus to not be part of marriage requires an explicit denial. For there to be no sex would require the sentence to say

but [Joseph] knew her not before she had given birth to a son; nor did he know her after the birth of her son. And he called his name Jesus. (Matthew 1:25)

Such a situation is so unusual that further clarification may be given along the lines of

and Joseph knew Mary not for their entire marriage, Mary remaining a virgin until her death.

As to the Lukan passage, there is nothing suggesting this is a vow. And the context reads that Mary cannot conceive because she is a virgin, not because she has taken a vow of virginity.

John is an argument against Jesus having brothers, not for Mary being a virgin. That is, if Jesus has brothers (Mary has children) that is evidence against Mary's virginity, but Jesus not having brothers is not evidence for her virginity.

Nevertheless, Jesus likely had brothers. These passages all mention Jesus' brothers:
Matthew 13:55; Mark 3:31; 6:3; John 7:3; Acts 1:14; and 1 Corinthians 9:5. Note especially what the people at Nazareth ask,

Is not this the carpenter's son? Is not his mother called Mary? And are not his brothers James and Joseph and Simon and Judas? And are not all his sisters with us? (Matthew 13:55-56)

These are rhetorical questions from people who grew up in the same town as Jesus. They are accurate, and the terms "brothers" or "sisters" can hardly mean "cousins" here.

This counts against Mary being a perpetual virgin (unless the siblings are Joseph's children from a prior marriage). Jesus being the firstborn and thus eldest would be responsible for his mother. The reason for asking John to take care of his mother may have been because at that stage even Jesus brothers did not believe him. In wasn't until after the resurrection that they had faith in Jesus.

What Scriptural evidence is there that Mary did not remain a virgin?

1. The evidence given above that Matthew 1:25 is contextually definitive evidence of subsequent coitus however we translate the preposition.
2. The evidence above that Jesus had siblings.
3. Paul's command concerning marriage in 1 Corinthians.

Paul commands men and women to give each other their conjugal rights. He specifically tells them not to abstain from sex other than for short periods of time for the sake of prayer. Conjugal rights are normative for marriage since creation and Paul reminds the Corinthians that this remains the case. It has always been important for marriage to have regular sexual expression. The command for Mary to not have sex with Joseph goes against not only the creation mandate, but also the specific command (a reminder) Paul gave to the Corinthians.