I've first heard of this custom while reading Marco polo's "Le livre des merveilles du monde" (The Travels of Marco Polo). IIRC the editor's notes, the custom did exist in some places of the Himalaya mountains.

> The guest may stay and enjoy the wife's society as long as he lists, whilst the husband has no shame in the matter, but indeed considers it an honour. And all the men of this province are made wittols of by their wives in this way.

Source: https://en.wikisource.org/wiki/The_Travels_of_Marco_Polo/Boo...

Western culture says it’s monogamous due to moral and law, but something between 1/4 to 1/2 of the men and up to 1/3 of the women “cheat”. So in terms of genetic diversity it’s the same, we just pretend it doesn’t happen while other cultures don’t see an issue.

There are huge differences in extra pair paternity across cultures and social class. People are not the same everywhere.

https://www.cell.com/current-biology/fulltext/S0960-9822(19)...

A Historical-Genetic Reconstruction of Human Extra-Pair Paternity

Specifically, we observe that estimated EPP rates among married couples varied by more than an order of magnitude, from 0.4% to 5.9%, and peaked among families with a low socioeconomic background living in densely populated cities of the late 19th century. Our results support theoretical predictions that social context can strongly affect the outcomes of sexual conflict in human populations by modulating the incentives and opportunities for engaging in extra-pair relationships.