A school, town, or nation making it mandatory or legally and socially acceptable for everyone in it to be naked all the time, which also facilitates spontaneous casual sex to happen in public spaces.

I've only found a handful of hentai manga that do that, and I've read them so many times that I can't get off to them anymore. The others either only have nudity with no actual sex going on, only have the girls be naked while the boys get to keep their clothes on, or a harem situation with only one guy getting laid while the rest get cucked.

Picrel is one example of the "no sex" nudist society type (sauce is "Nuno busoku no sekai" or "A world without clothes" by Mushoku Miho). The most that happens is the redheaded girl masturbates on the rooftop after seeing a boy in her class get a hard-on and ends with her being caught by a group of boys with the last page just showing her fainting from embarrassment (said boys don't even get hard after seeing her squirt!)