But let's say there are grounds to condemn the theology on the Protoevangelium. Still, it does nothing to take away from the fact that a 2c. book recorded Mary's intention to not consummate her marriage to Joseph carnally, even before the Annunciation.
It appears you are assuming the "fact" of her intention to remain a virgin without evidence.
This story was written over 150 years after the facts, most likely to insert perpetual virginity belief where the bible seems to oppose it. It is not surprising you find validation in this fanciful apochryphal myth rejected by Jerome and called "apochryphal ravings" by Aquinas.
We don't know who wrote it (written around 150 AD) but it certainly was'nt James the Lord's brother, so the named author is a liar, it also says Mary lived in the Holy of Holies from age 3 to 12 being fed by an angel among other fantastic and false claims.
While an immediate answer is that our knowledge of Joseph comes from the Bible, it is not difficult to see that the Scriptures make no mention or implication of Joseph's advanced age, or other similar details. Such particulars are imaginatively supplied by certain apocryphal writings. Though non-canonical and never considered historical by the Church, such writings have had a great influence on popular devotion.Originally written around the second half of the second century, the aim of this book is to glorify Mary, which means her virginity must be reconciled with the Gospel phrase regarding Jesus' "brothers."
What weight should be given to these texts? From apostolic times, Irenaeus considers that "apocryphal" means "forged" and Tertullian considers it synonymous with "false." Mary and Joseph are made into leading characters, rather than supporting participants in the great mission of Christ. The purpose of these works is apologetic, doctrinal, or simply to satisfy one's curiosity. Though they have a certain literary worth, their stories are much too fantastic to be given historical value in their own right. A decrepit widowed Joseph does not seem capable of performing the role of husband, father and protector that is clearly ascribed to him in the Gospels. Jerome and a number of the fathers flatly rejected the central assertions about St. Joseph found in The Protoevangelium of James and in the other apocrypha which build on it.
http://www.osjoseph.org/stjoseph/apocrypha/
All four Gospels were written anonymously, and between 40 and 50 years after the events. The Church assigned authorship to each Gospel by the end of the 2nd century "according to tradition" (read: legend, or myth). And yet, all Christians accept it on not much more than blind faith, even if they reject the Church!
And while alleged Mary's abode in the Holy of Holies may sound a little over the top, it's not as if the rest of the Bible stories lack in that department to a much greater degree!
Your flat out rejection of Mary's story in Protoevangelium is based exactly on what objective criteria? It's not as if we have some independent corroborating evidence to come to such a conclusion! The best you can do is say that it's "not "inspired". First, how do you know that it isn't, and second, truth doesn't have to come from "inspired" works. The Book of Enoch is not, and yet Jude quotes from it as truth.
People believe what they want to believe because it makes them comfortable. That's what faith is all about—a self-derived comfort from a conviction that something is true, like saying "I am saved," or "he is in a better place now," etc. You can't prove it false and you can't prove it true, even if it seems fantastic. You can either accept it or reject as a matter of faith, but never as a matter of fact.
Any book form 2c. that says something to that effect is evidence. It is not a direct scriptural evidence, or archeological material evidence, but evidence it is. Since the book survived and even an apostolic provenance was (incorrectly) attached to it shows that its material was bleievable to the people close in age and culture to the times when Mary lived.
You can see that Mary did not intend to have children from Luke 1:34, "How shall this be done, because I know not man?". A woman intending to have children with her future husband would not ask how children are made.