Stories set in a world where the male population is significantly lower than the female one. Mostly wish fulfillment/self-insert type stories, with an isekai'd male protagonist who effortlessly builds a harem. Though some of the better ones do explore the societal impacts of the imbalance.

Common tags: male protagonist, harem, polygamy, sexually active protagonist, devoted lovers, s*x friends, in*est, gender reverse roles, matriarchy, isekai, reincarnation

[Created this list so people can more easily find this kind of setting as there's no tag for it. As such, do not take these as recommendations. If you want more similar series I suggest look at rec lists for 'Gender Reverse Roles' as there's a lot of overlap in themes.]