Good observation about the lack of bare-chested men after the comics code. I'm not sure whether the comics code had anything about that or not (though I tend to doubt it got that specific).
But I have heard horror stories from artists who got changes back from the CCA censors; some of the changes they demanded were not things that were specifically mentioned in the code. It was just that particular censor's interpretation and opinion.
So it might be the result of the censors being inordinately picky, or it might even be that the artists began to play it safe so they wouldn't have to keep revising their work.
Link to the book:
Thrilling Comics 04