“You want to make sure that you narrate what is going to be happening,” a blond woman in a skintight nurse’s costume said. She had just demonstrated how to safely, and consensually, stick a willing partner with hypodermic needles.

The subject of her class was “medical play” and the crowd was standing-room-only. The event was hosted by the Eulenspiegel Society in Manhattan, which describes itself as the “oldest and largest B.D.S.M. support and education group” in the country.

The “nurse,” Margot, was not acting as a health care professional, though she did offer hygiene tips. She was there, with her role-play partner for the evening, June, to model best practices. (Many of those interviewed for this piece, including Margot and June, did not want to use their full or legal names for fear of stigma.)

“You create a container for the things that are your worst fears, your darkest fantasies, and you create very strong boundaries around that,” Margot said. “Respecting those boundaries is the most important thing.”