Even if true, (which I do not grant), how does that explain the same beliefs in all the rest of Christendom outside of Rome? (Byzantine, Palestine, Africa)
You do know that Luther and Calvin also believed in the perpetual virginity of Mary?
And Luther said:
". . she is rightly called not only the mother of the man, but also the Mother of God. . . . it is certain that Mary is the Mother of the real and true God."
"The veneration of Mary is inscribed in the very depths of the human heart."
John Wesley:
"The Blessed Virgin Mary, who, as well after as when she brought him forth, continued a pure and unspotted virgin."
You guys have no idea how unhistorical you are, do you?