Hey, finally something where I actually have a somewhat decent story! (Not so much of a story as a situation but..)
During my years studying I participated in several parties where part of people where more or less naked but this one particular one stands out as one of few that I've later told stories. There was a pool, plenty of people opted to go there even though they didn't have swimming clothes along. It wasn't as much about how many people I saw naked (15-20?) and who or how many saw me naked but the uniqueness and relaxed atmosphere. Plenty of people in their early/late 20's drinking alcoholic beverages, roughly as many men as women, chiling out at pool, some playing with beach ball, mother with a baby, some singles but mostly couples. Really only thing that later bothered me a bit is that even though I was naked maybe slightly over hour I still managed to miss the naked bride in her wedding party. :-( At least I have story about seeing naked groom on his wedding but for some reason it doesn't sound as much fun. (I'm a male)
Weirdest wedding setting I've ever been to. Not traditional wedding by any means as there were no church, priest or anything like that. But they had registered their marriage very recently and as much as I remember this was marketed to me as "(partially wedding) party".
Oh yeah and since this happened in finland (yes, I deliberately "forgot" to mention that there was a sauna by the pool to make my story slightly better) it wasn't that much out of norm to see naked people really because that is what you do when in sauna. (Well, even on finnish scale the setting was a bit weird.)
TL;DR OP deliberately forgets many things. Like that time in army where there were like 30-50 people in one huge sauna. But that story could be told by 70% of all man in finland. And they all were men there. So really a non-story.
On one summer job i liked similar setting better as 90% of people were young (18-25) women and I was one of the few men in sauna. (and sadly 95% of women had their swimsuit on. Why? Most there were women, women!) That was more of a awkward situation as I was one of the few naked there...