It's called ACTING, dummy
Sometimes in movies a more experienced method actor will try to get a more natural reaction out of an inexperienced actor by throwing him/her a "curve". For instance, Robert Deniro once famously changed a line in "Raging Bull" and asked Joe Pesci if he f^cked his mother. In "Cape Fear" Deniro unexpectedly put his greasy thumb in a teenage Juliette Lewis' mouth. Of course, Pesci COULD have sued Deniro for creating a "hostile work environment" and Juliette Lewis COULD have charged him with "sexual assault on a minor" (instead of fellating the thumb like a little slut), but the point is they were ACTING. Deniro did not do these things to exert personal power or for his own sexual satisfaction, but to MAKE THE SCENE MORE EFFECTIVE. Similarly, movies frequently have fairly realistic fight scenes where actors make real physical contact with each other, but that doesn't make them assault and battery.
The highly inexperienced Maria Schneider may not have consented to exactly how the scene was filmed, but that doesn't mean she was raped if Brando didn't actually sodomize her. She's an ACTRESS. She consented to be naked. She consented to filming sex scenes, and even a previous rape scene. If the butter scene was not filmed exactly as scripted or like she wanted, it was not very nice of Brando and Bertolucci, but it wasn't rape. It wasn't like they came up to a random 19-year-p;d girl on the street, pulled her pants down, rubbed butter on her butt, and pretended to sodomize her. Obviously, THAT would be sexual assault, even if there was no penetration. This was a MOVIE SCENE though, and it would be a dangerous precedent if actors could charge each other for assault if they deviate from the strict, scripted choreography of a sex scene. Of course, an actress can be RAPED, even in a porn movie, but that just isn't what happened here.
The main point you can't expect a 1972 movie to be filmed the way a date is conducted on a liberal PC US campus circa 2016 with the actor asking the actress' permission every step of the way. Obviously, young actresses have the right not to be raped on a camera, but they don't have to "consent" to every aspect of how a movie is filmed. They probably should have involved her in the decision and they obviously disrespected her acting ability, but it's still ACTING, not rape.
"Let be be finale of seem/ The only emperor is the Emperor of Ice Cream"