An important historical document which supports the teaching of Mary’s perpetual virginity is the Protoevangelium of James, which was written probably less than sixty years after the conclusion of Mary’s earthly life (around A.D. 120), when memories of her life were still vivid in the minds of many. http://www.catholic.com/tracts/mary-ever-virgin
The perpetual virginity of Mary has always been reconciled with the biblical references to Christ’s brethren through a proper understanding of the meaning of the term "brethren." The understanding that the brethren of the Lord were Jesus’ stepbrothers (children of Joseph) rather than half-brothers (children of Mary) was the most common one until the time of Jerome (fourth century). It was Jerome who introduced the possibility that Christ’s brethren were actually his cousins, since in Jewish idiom cousins were also referred to as "brethren." The Catholic Church allows the faithful to hold either view, since both are compatible with the reality of Mary’s perpetual virginity. http://www.catholic.com/tracts/mary-ever-virgin
(we see the influence of the apocrypha in the above and why it should not be considered sacred text. it also completely ignores the context of the passages that clearly identify Jesus having siblings.)
However, there are problems with this writing.
The Protoevangelium of James is like other forgeries trying to capitalize on an apostle. James, the half-brother of Jesus, was elevated to an apostle after he saw the resurrected Savior (Galatians 1:19; 1 Corinthians 15:7). So some people thought using his name would give some much needed credibility to the book. However, the church rightly recognized this book was not from the Apostle James. The early church father Origen wrote a commentary on Matthew in which he rejected The Protoevangelium of James as spurious and affirmed Mary had other children.3
The concept of Mary’s perpetual virginity is conveniently explained in The Protoevangelium of James since James is viewed as an older step-brother brother of Jesus being a child of Joseph and his first wife, prior to his marriage to Mary. However, there are a number of mistakes in this book and statements which contradict the Bible that an apostle writing under the inspiration of the Holy Spirit would not make. https://answersingenesis.org/bible-characters/is-the-perpetual-virginity-of-mary-a-biblical-view/
Contradictions between this writing and the Bible.
Protoevangelium calls Gabriel and archangel which was a common designation for Gabriel in apocryphal literature written after the first century.
The Bible only calls Michael an archangel.
Mary's reply to Gabriel is different in the Protoevangelium: "What! Shall I conceive by the living God, and bring forth as all other women do?"
Her reply in Luke: “Then Mary said to the angel, ‘How can this be, since I do not know a man?’” (Notice the contextual use of the phrase I do not know a man...Mary had never had sexual intercourse. The only other reading would be to infer that Mary did not know any men in her life which would be ludicrous.)
The Protoevangelium notes Jesus was born in a cave outside of the city of Bethlehem.
The Bible notes Jesus was born in Bethlehem, the city of David.
The above was provided from https://answersingenesis.org/bible-characters/is-the-perpetual-virginity-of-mary-a-biblical-view
There are other errors in the Protoevangelium when compared to the Bible, but we've illustrated why we shouldn't take this work to be of value.
Catholic teaching on Mary is predicated upon this work, as are some of the ECFs based their works on this. Sadly, catholic teaching has become so entrenched on this topic that even with Biblical evidence to the contrary, they cannot back off this without the remainder of their teaching being called into question.
This is why you compare these extra writings with the Word to see if they square away with the Word.
This is why you stick with the Word and not the extra writings.
It's why you don't appeal to John 21:25 to allow these false writings to be elevated to the status of legitimacy.
It's why the Bible should be our final and ultimate source for these matters.
It’s why the Bible should be our final and ultimate source for these matters.
By just reading the Bible it sure looks to me like Mary only had one child which was Jesus.
“It’s why the Bible should be our final and ultimate source for these matters.”
WRONG.
The first theologians, the early Church fathers used the oral tradition, and for years discussed what should be included in scripture and what should be omitted, they based this on tradition and custom and liturgical practice. The Bible as you should know were books that did not fall from the skies and self-arranged themselves in the order in which they are assembled.
Those who did so, acted with the Petrine authority, what more supreme authority does one have than to actually put together the authentic words of the Bible? But yet this not all. Included in these works were what were not written but were handed down as part of the deposit of faith to the first holders of the Petrine ministry. John 21: 25 refutes the nonsense that the Bible by itself without regard to other forms of scriptural interpretation is the ultimate source of all belief:
“But there are also many other things which Jesus did; which, if they were written every one, the world itself, I think, would not be able to contain the books that should be written.” (Douay-Rheims Bible)
Thus interpretation must be based on the written word, on the interrelation placed on them by those who assembled these texts, and corroborated by liturgical practice and forms of worship and veneration believed by the early disciples of Christ.
This has been debated by theological scholars ever since for 2000 years by both Catholic, and by non-Catholic theologians who have converted to Catholicism. They all agree Christ founded ONE Church with ONE truth to be carried forward with the same Petrine authority given to the early Church fathers.
Those who refute the doctrine of the incarnation and the Holy Eucharist have demolished the very cornerstone beliefs of Christianity and in a word have no right to call themselves “Christians” except as well-intentioned but just misguided preachers that runs the whole gamut from Jim Jones and David Koresh to the TD Jakes, Al Sharptons, Joel Osteens and Billy Grahams. Theirs is the interpretation of simpletons rejected by a vast body of authority.
Simply throwing out swatches of scriptures is old hat that has long since been discarded and relegated to the trash bin of inauthentic interpretation shorn from context and lacking historical support.