man, I gotta say it seems to me sex-phobic to stress out about whether or not a boy is gonna see a mature man naked. Im sure that's gone on throughout history without any repercussions. Of course Im just talking about SEEING a naked male body -- am NOT condoning any kind of eroticism.
I know when I was a kid, I was curious to see naked adult mens' bodies -- not from lust, but from an innocent desire to see how my own body would mature. there was a time when I was very confused about this,and it was truly interesting and helpful to know. As I started adolescence, the changes werent surprising -- but rather expected, and exciting. Never had any misconception anything was "wrong" with me physically.
Flip side: don't you think, if boys were deliberately prevented form ever seeing adult men nude, they might grow up feeling there was something inherently shameful about it?