An open relationship is a subset of
polygamy, a system in which
romantic partnerships are not exclusive - i.e. in any one relationship either partner might have other partners of either sex, and their
status in their lives will be equal all round. Most people in polygamous relationships retain what is called a "primary partner", usually someone to whom one is legally
married.
Not living in small, tight-knit communities makes polygamy in its primal state pretty much impossible because of various legal issues and problems with the upbringing of children.
I believe in polygamy, I think it's a system in which many of the problems people have in their monogamous relationships - jealousy, possessiveness, boredom - are eliminated. Unfortunately we are brought up from infancy to view things very differently, and the paradigm shift a polygamous relationship requires is too much for most of us to make. I know people who are in different kinds of poly relationships, and only the most careful, mature ones succeed. If you treat it as an excuse to sleep around, it's doomed.