Then I remembered Bertolucci. He's a very minor character in this book, but his history is relevant to the point at hand. When Last Tango in Paris came out, a lot of people freaked out, especially over the butter scene. The film was censored in more than a few countries, and Bertolucci faced legal problems in Italy. People argued and debated the graphic nature of the film. Even today, there are segments of the internet that still think the sex in that scene was real, though it looks less graphic thanks to HBO.
Maria Schneider had a lot to say about it after the fact. I can see her point of view, but I think she used public ignorance to get more sympathy. She knew that all of the sex was simulated, of course, but she hated the butter scene. By insinuating that it was unsimulated, she could get people to accuse Brando, and mostly Bertolucci, of rape. And that's not right. Bertolucci humiliated her, on purpose, but Brando's penis never went anywhere it should not be.
I suppose it's good that people think sex scenes are real. It means the filmmakers are doing a good job. If the audience knew just how unerotic shooting a sex scene was, they would never get excited about what the characters are doing.
But then you have to wonder, do people think fight scenes are real? When Sean Bean dies onscreen, he doesn't really die. He's very much alive.