And you know this, how? I've had it pop-up on me for no particular reason as, I suspect, most men have.
My rule is simple-- if you want to avoid a potentially embarrassing public display, (a)don't strut about nekkid and (b)wear pants or shorts with ample expansion space to hide it.
You know, the funny thing is this: I've been to many nude beaches many times back in the early 80's and 90's, when I was in my "prime" and never got a "pop up"
Too many people think that going to places like that are about perversion rather then knowing it is just about experiencing the freedom of it.
It's not for everybody, but then again those who choose not to participate are free not to go.