Almost, kind of, maybe ...
By the time the Proto-Gospel was written, I suspect the story had been known among Gentile Christian hands for a good number of years. And, as is in it's XXX-rated version, I can easily imagine that it was never told by the Jewish Christian elders.
It may seem strange to say so here, but I'm inclined to imagine that what occurred was a variant of Hoyle's law. Hoyle's law says: “By the time you understand the rules of a complex game, you will no longer be able to explain those rules to anyone who does not already understand the game.”
The Jewish Christians may well have understood that the "defilement of Eve" didn't involve any sexual activity but the Gentile Christian newbies surely wouldn't have know that for certain. And here, I think, the far more important "function" of the story comes into play. Without the XXX-rated version of the story, the notion of imputed or ancestral or original sin is really pretty weak.
However, roll the XXX-rated version of the story out on the stage in a Beginner's catechism class, and then tell the students: "... and that's where Original Sin comes from. And, by the way, that's why the snake is a reptile that crawls on its belly while other reptiles travel on four feet."