How do those who support the perpetual virginity of Mary doctrine harmonize the Protevangelium of James with the Gospel of Matthew?

The Protoevangelium of James, is an apocryphal Gospel probably written about AD 145. The document presents itself as written by James: "I, James, wrote this history in Jerusalem." The purported author is thus James, the brother of Jesus, but scholars have established that the work was not written by the person to whom it is attributed.

Before the end of the second century there is universal acceptance of the four Gospels, Acts and twelve of the apostle Paul’s letters making up the canon of the Christian Greek Scriptures. (Only a few of the smaller writings were doubted in certain areas.) They included: Muratorian Fragment, Italy AD170 Irenaeus, Asia Minor AD180 Clement of Alexandria AD190

The book of Matthew, in complete contrast to the Protevangelium of James, '...is inspired of God and beneficial for teaching, for reproving, for setting things straight, for disciplining in righteousness, so that the man of God may be fully competent, completely equipped for every good work. (2 Timothy 3:16, 17)

Those words are often correctly quoted to show that the Bible is God’s inspired Word. But what was Paul’s purpose in writing them? Paul was speaking to an elder, one with the responsibility to ‘reprove, set things straight, discipline in righteousness,’ within the congregation. Hence, he was reminding Timothy to trust the wisdom of the 'inspired Word', in which Timothy had been instructed since infancy. The lesson is clear for us also.

Your question is:

How do those who support the perpetual virginity of Mary doctrine harmonize the Protevangelium of James with the Gospel of Matthew?

It feels like there's an implicit assumption that those who support the Perpetual Virginity of Mary must follow the concept from the Protoevangelium of James that Joseph was an older widow with children from a previous marriage.

There's actually a different belief that many supporters of the Perpetual Virginity of Mary hold, which is that the "brothers" of Christ were cousins (rather than Joseph's children from a previous marriage).

This belief shows up in the Fragments of Papias section 10 (~100 AD) , and is first clearly articulated as an argument in Jerome's The Perpetual Virginity of Blessed Mary: Against Helvidius. section 15 (383 AD).

In brief, the belief goes like this:

• John 19:25 tells us that Mary mother of Jesus had a sister named Mary of Clopas (who was Jesus' aunt)
• Mark 15:40 (and Matthew 27:55-56) tells us that Mary of Clopas was mother of James the less and Joseph.
• Jude 1 speaks of "Jude, the brother of James". This is likely the Jude who is son of Mary of Clopas, brother of James.
• Matthew 13:55 identifies James and Joseph and Simon and Judas as 'adelphos' of Christ. We see James and Joseph are his cousins through Mary of Clopas, so it follows that Simon and Judas are likely also children of Mary of Clopas.
• This wiki lays out the full biblical case, supplementing it with historical records.

So to answer your question, many of those who support the perpetual virginity of Mary don't bother attempting to harmonize the Protoevangelium of James with the Gospel of Matthew. We see the Protoevangelium simply as an uninspired, though respected, ancient church document that we don't need to harmonize with the inspired Gospel of Matthew.

The Protoevangelium of James may crystalize and record early, and obviously accepted, beliefs about Mary. But they are not 'where we get the doctrine from'. They are not Scripture. Similar to the book of Enoch, contaning the prophesy quoted in Jude (Jude 14-15), while not being part of Scripture.

The fact that it was written that early, and is trying to be an authentic and believable document proves that only those facts about Mary already accepted and believed would be developed or explained. I think this is the real strength of the document and its usefulness. It proves that the readership already believed at least that she was x, and the document simply attempted to detail x. Not introduce anything necessarily contrary to the common belief.

However, Luke 1:34 is enough to prove Mary intended to remain a virgin, even when already married to Joseph.

The harmonization would be that the Gospel of Matthew is divinely inspired Sacred Scripture, while the Protoevangelium of James is merely pious tradition. In case of nonconformity between the two, Matthew takes precedence.