Touching someone's anus without their permission is sexual assault
Yes, the actual rape in the scene was simulated and both actors knew about it. The actress was not raped, since that part was acting. I get that.
But Brando did put his fingers on her anus without her permission, since that was the part she was not told about. Since she was not told about that, she obviously did not give her consent to that. Therefore, Brando sexually assaulted her with his fingers.
The director was also partly responsible because he planned it all out.