I have shown before how Japanese men, particularly younger men, who gather in groups for sports and athletic events, often change in and out of the athletic gear in the open - sometimes plainly visible (although not enough so) to the casual bystander. I've run across those situations many times and even documented them here on this blog (Search for: Japanese / frisbee / soccer )
Here are a couple of different situations where Japanese men get naked (or partly so) together in public or socially.
1) Hot springs (onsen) or public baths (sentou)
The top photo is actually as a screen-shot taken from a television program. In Japan, there is not distinction between the prohibition of male or female nudity on TV, film or print media.
In NONE of them can gentials (sex organs) or public hair be shown. However, simulated sex, buttocks, breast, and full body nudity (including blurred out or mosaic coverage) of sexual organs can be legally shown on TV, movies or in magazines or advertising. The guy above is a Japanese 'talento' - a actor/comedian/musician type who regularly appears on TV and in movies.
I've been to a Japanese bath house in Japan, and it is REQUIRED that you wash your entire body thoroughly. That includes all the orifices. Many guys do it twice. Then rinse off and maybe do it all again a third time if you're really dirty. Only then are you allowed in the soaking tub.
My own experience with public baths and hot spring resorts:
There used to be one in every neighborhood. Even in a city as large as Tokyo, there were many people who lived with a bathroom in their apartments (they did have a separate toilet, of course). So it was customary (and still exists today but to a far lesser extent) for young and older men (who lives alone or in worker's dormitories) to go to the local public bath. I used go too - to get clean and have a relaxing soak in the large
communal tube. But it was also very erotically stimulating to catch glimpses of all the scrubbing of backs, and chests, and the cleaning of cocks and assholes. It wasn't only one that I (or another guy who was also a little too eager or interested) got a semi hard-on while sitting across from another young stud - whose toned muscles were the results on many months of day labor. I love it best when 'buddies' would scrub each others' backs, or joke about the size of his own or poke fun at the 'large' size of his friend's dick.
Japanese festivals or matsuri -- These are the seasonal outdoor and street festivals that derive from the traditional Shinto shrines celebrations related to nature its relationship to humans, mostly agriculture such as planting, harvest, seasonal changes, etc). Most of these o-matsuri involve groups of men (sometimes mixed groups with women, too) carrying very heavy portable shrines (o-mikoshi) and they usually involve some jostling or competition between the separate groups involved. A theme of virility and fertility the male's role as 'planter of the seed' - meaning both farmer of the fertile soil and also plowing his blunt instrument in the folds to the entrance of woman's womb are symbolized by various actions and artifacts used in these social rituals, dating from a few thousand years back - when Japan was a completely agrarian society.
In these shrine festivals (o-matsuri), it's traditional, or at least used to be more so, to wear a 'fundoshi,' the wrap-around and tied underwear (cock & butt strap).