In front of a crowd: Nervousness-inducing. A bit vulnerable. Distracting. A little scary. Sexual intimacy creates vulnerability, and feeling that vulnerability in front of an audience of strangers is a little…well, disconcerting.
Oddly, though, it’s less so than having sex in front of people who are friends or acquaintances but not lovers. When the people watching aren’t people you know, you still experience that vulnerability, but there’s also a distance. When I did this at a BDSM convention, I knew these were not people in my immediate social circle; at the end of the convention, I would not likely see them again.
But when it’s in front of people you know, you’re inviting them into that intimate space in a more immediate sense. It can change the nature of the friendship; in particular, I’ve discovered that having sex in front of people I’m attracted to and engaged in an ongoing basis with can open the door to emotional intimacy in much the same way as having sex with them would.