This is interesting, Frank. It may be because they don't spend all their time in Pagan and/or nudist environments, but are also exposed to more traditional people at school, Grandma's house, etc. They are told when and where they can be naked, and maybe it just gets too confusing for them, so they stay covered up all the time.
Another thing I've noticed is that heterosexual men can't seem to stand the sight of another man naked. My sons hate to see their dad naked. When I was a girl growing up, I saw my mom, sister, and grandmother naked all the time, and it never bothered me a bit, even after I reached puberty. Why is this?