Sex work breaks down into two major categories: sensation or spectacle. It's either something where everyone has the option to physically participate, or it's a look-but-don't-touch situation. I recently spent an unusual—for me—evening at the New Horizons swing club in Lynnwood, and it struck me that public BDSM parties and swing parties diverge in an oddly similar way.

Sure, the activities are different. Swingers fuck. People at BDSM parties don't. (Okay, occasionally they do, but not very often.) Still, the gatherings do have some comparable features. Each is a group of sexually alternative people getting together to participate in what turns them on. They may come with their partner of choice—just for the excitement of being in novel surroundings and watching and being watched. Or they may be there to seek new partners. Or both.

Contrary to my expectations, I didn't find the overall feel of the swing party markedly different from a BDSM party—at least, not in the social area. But when I walked through the designated sex area, I noticed one big difference: The swingers would often look up and make prolonged eye contact with me as I watched them fuck.

That was startling. It's rare for people doing BDSM at large parties to make eye contact with anyone outside their scene, and it's considered rude for onlookers to try to catch and hold a player's gaze. You watch, of course—but you don't stare. And no one in a scene would stare back at you.

I wondered if, in the swing world, prolonged eye contact constituted an invitation to join in the fucking. My swing-etiquette expert, official New Horizons host Sam, said not necessarily. "The rules are: You ask. Eye contact is simply the first stage. If there seems to be an invitation, a common second step is to move closer. That gives the couple the opportunity to reach out to you, either verbally or physically. (Don't stand over them, though—I find that creepy.) But I would never recommend that the third person actually walk over and touch the couple without asking."

Those sound like reasonable rules to me. But still, it felt odd to have someone looking back at me. That evening, I watched a man do a series of slow push-ups into a woman, who seemed to be enjoying the deliberate strokes. With each thrust, he glanced down at her, but as he pulled back, he locked eyes with me. Maybe he meant it as a friendly invitation, but it confused me, and I walked on rather than risk being misunderstood. It's extremely rare for a stranger to get invited into a BDSM scene already in progress—watching people play is like watching performance art. It's just understood that I am the audience, not a potential participant. If I went back to a swing club, I'd have to get used to the idea that swingers have a much more permeable division between spectacle and sensation.