A guest lecturer in the Feb. 21 class had been discussing bondage and sexual fetishes in a human sexuality class.
Published reports say students were invited to see a live sex act after class, and about 100 stayed to see it.
Northwestern President Morton Schapiro strongly criticized the lecturer.
"I feel it represented extremely poor judgment on the part of our faculty member," Schapiro said a statement issued by the Evanston university.
Schapiro said it did not matter that the incident occurred after the class was over, that attendance was voluntary and that the students were warned of the explicit nature of what they were about to see.
The Associated Press contributed to this story.