Just saw a reddit post where a guy was divorcing his wife of 12 years because she suggested it, and it gave me the idea for this question.
Wikipedia defines: "Open marriage is a form of non-monogamy in which the partners of a dyadic marriage agree that each may engage in extramarital sexual or romantic relationships, without this being regarded by them as infidelity".
To be honest, although my wife and I don't exactly have a straightlaced relationship, I have my doubts about a full on open marriage working out in the end. Not saying it can't work out, but in my opinion it needs almost flawless planning and an emotional maturity and detachment that are very rare.
The last modern proponent of the practice was Martin Bormann, Hitler's secretary.