American culture... I think [stealth] (especially those who are devout religionists...)
There is an extreme sense of shame with the human body for many people here in America. For me... it is no big deal. I have showered in the locker-room with many men. Like you said... a penis is a penis.... who really cares.
The first time I met my wife's uncles they were all standing naked after getting out of the sauna (Finland). They were nice guys. Nice... nude guys. But nice just the same.
Shame breeds mental abnormalities... such is life for many.