Two nations, one of all males, one of all females, on some occasion, they get together to produce children, both sides crossbreed for the sake of it, later, the children are given out to each of the nations according to their sex, before the nations separate again to continue until the next time arrives for their needs of producing another generation.
Otherwise, any man who set foot on female nations land or any woman who se5 foot on male nations land is killed on sight.
Both nations are made of a higher caste that serve as warriors and philosophers and kings/queens, while the rest of the society is propped up by slavery (think exactly like Sparta, but with only one sex).
There's also homosexuality, a lot of homosexuality, which frequently takes the form of older mentors being partnered with their younger students. Pederasty or Korephilia, depending on the national sex. Yes, both nations are full of pedophiles.
