Here's something I read on rec.nude decades ago, it's always kind of stuck with me:
"This is a common question among newcomers to nudist activities. In practice, it is nothing to worry about. What usually happens is something like this: You get an erection, somebody notices, points at you and yells, "Hey, look at the hard-on on that guy" or, "Look at that guy trying to hide his hard-on". Then everyone gathers around, pointing and laughing. If you try to run away, they all follow you. People start taking pictures. Eventually, some guy thinks he catches you looking at his girlfriend/self and beats the hell out of you. After this happens two or three times, you get conditioned to always go limp when you see a naked body."