Wasn’t the Protovangelium deemed apocrypha? In my opinion—the message of Jesus (Joshua ben Joseph) isn’t diminished by his having brothers and sisters. In fact, I respect him more. Even if he was married—doesn’t cause me to re-think his message of love—If he could do all he did—save mankind and have a wife too—that would prove his godhood.
The PoJ was not included in the Canon of Scripture because it contained errors. That it was considered means that it contained some truth. (An interesting side note is that this, along with numerous other works accredited to Apostles and Disciples, presented the problem that a formal Canon satisfied.) Those of us concerned with accurate hermeneutics look to a number of sources to understand the original intent. Agricultural archeology helps us to understand references to people who lived close to the land. History and archeology help us to understand the geopolitical context of events and pressures on peoples. Contemporaneous writings like the PoJ give us clues into the culture and interpersonal relations of the peoples of the times. All of this is necessary because presumed knowledge of the day does not always survive 2,000 years and multiple translations.
This particular example is important to Catholics because Tradition is important to us. In contradiction to Protestant teaching that Catholics must choose between Scripture and Tradition, embracing one OR the other as Revealed Word, Catholics embrace BOTH Scripture AND Tradition as comprising the Sacred Deposit of Faith. This Sacred Tradition from which the written Scripture of the New Testament was derived has always taught certain aspects of the Blessed Virgin Mary not specifically addressed in Scripture. From Tradition we believe in the perpetual virginity of Mary. Sacred Scripture, being silent on the issue, does not contradict that. Contemporaneous writings and the subsequent writings of the Early Church Fathers whose life times overlapped Hers, and in some cases actually knew Her, affirm the Tradition. However, proof is not needed. Faith is what we believe in the absence of proof.
Peace be with you.