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.
I do not consider a "pop-up" when not intended nor the lack of a "pop-up" when intended to be an abnormality. From what I've read, it is the rule with most men, which is why (b)wear pants or shorts with ample expansion space to hide it is usually a good idea and (a)don't strut about nekkid [unless you are with your wife or in a private situation where a pop-up is wanted] is always a good idea.
Even if you can't control what Mr. Willie does at any particular time, you can control how he is displayed.
A "pop-up" has to do with blood flow and may not even be sexual at all. There are scores of reasons ranging from sudden cooling, REM or dream patterns, etc., etc.
If you have total control of Mr. Willie then, good for you, but most of us don't. Responsible dog owners don't turn their pit bull loose in the neighborhood. They keep it on a chain, behind a strong fence, etc. Same with Mr. Willie.