In the 1960s, a rather obscure anthropologist by the name of John Cowan Messenger discovered an isolated and very small community in a place called "Inis Beag" in Ireland. This was apparently a false name and the place is in reality called "Inisheer", they deliberately misnamed the area to "protect the people".
"Inis Beag", according to the anthropologist, was the most sexually repressed in the world. Amongst other things, masturbation and premarital sex were unknown and people had sex while fully clothed.
I need not describe in detail the practices of this community, one may see them in detail at: https://en.wikipedia.org/wiki/Inis_Beag#Sexuality
Another community discovered in the Cook Islands, named as "Mangaia", were named as the most sexually liberated. They were the total opposite of the people in Inis Beag. Casual sex in females was rampant and sexuality freely defended and nudity the norm. You can get the details of this community and Inis Beag on: https://en.wikiversity.org/wiki/Cultural_differences_in_sexuality
These two places have been many times mentioned together in many anthropological works and popular culture. However, the person who detailed the sexual culture of Mangaia was Donald Marshal and he was the only person who did so. All studies afterwards merely cite him. In other words, there's no other reference on Mangaia. An extremely similar situation prevails for Inis Beag, where Messenger is the only source.
Inis Beag and Mangaia are held as total polar opposites. The former with the most sexual repression, the latter with the most sexual liberation.
How true are these claims exactly? They have been often tossed around in forums and other places but nobody it seems has actually investigated their validity.