Reverse cuckold, any known case?

Do you know about any couple in which the female enjoys and encourages her husband to have sex, date, seduce and flirt with other females,while she is willing to stay monogamous?

i.e.why doesn't it even exist?
 
There are entire religions based on that. The taboo on cuckoldry has many faces, but one of its historical roots resides in the uncertainty of paternity. Before DNA tests and contraception (which are pretty modern things) a women could cheat, and make her husband raise the offspring of her affairs, while a man could never just have a kid with other women and trick her wife into thinking it hers.
 
I meant, it does exist, is quite widespread, and there are even many social groups that take it as a norm, but we just don't see it as a taboo in the same sense of the hotwife thing.
 
I knew a couple that they had a regular "girlfriend". They even went on vacations with her, or dinner, and while at the time we only knew her as their friend, later we learned they spent most night all three together.