A few Fridays ago, a young contemporary dancer named Will Rawls was working at his current production, the Marina Abramovic performance art retrospective at the Museum of Modern Art. Specifically, he was standing naked in a gallery entrance, facing a naked woman, as museumgoers passed through the narrow space between them. It was a re-enactment of “Imponderabilia,” a well-known piece originally performed by Ms. Abramovic and a partner in the 1970s.

Out of the corner of his eye, Mr. Rawls noticed an older man preparing to walk through.

“He proceeded to slide his hand onto my ribs and back and then touched my butt,” Mr. Rawls said. “As he was passing me he looked me in the eyes and said ‘You feel good, man.’ ”

“I just turned and looked at the security guard and said, ‘This man is touching me.’ Then I looked back at my partner and left it at that.”

When his shift was over, Mr. Rawls said, he learned from a security official that MoMA had revoked the man’s 30-year membership and barred him from returning to the museum. (The museum would not comment on specific incidents, but issued a statement saying that “any visitor who improperly touches or disturbs” a performer will be removed.) It turns out a crowded museum, like a crowded subway, is no excuse for an improper touch — a lesson that has been learned the hard way by some visitors to the retrospective, “The Artist Is Present.”