I've met some couples that don't have this dynamic and I admit its kinda fascinating lol. But yes, it's rare for it to be equal.
I met a London couple that I posted about recently. They are married and each have 3, or so other lovers to see during the week. All lovers are introduced to the husband/wife at some point.
When they celebrated their 10 year anniversary abroad in a lavish ceremony, they invited their current and some ex lovers to celebrate with them. That is a couple that truly are cool with an open relationship
. Another girl I know was swinging and open, but I can tell she loved the extra attention and freaky sex.
As for women that put up with cheating on a regular to the point it's really an open relationship... I don't have many people around me like that. The majority of my close friends are with men that strive to be faithful/no drama.
One of my old bffs hangs around in circles where this stuff is everyday. Most of her friends and family date these dudes, so in her opinion all men are like this. Because I don't have this experience it hasn't crossed my mind to consider an open relationship when its not my preference, but I can understand why she's given up on the idea of a man with monogamous intentions. When people look around them and see that what they want is unlikely, thats when they give up and allow things to be more flexible. ALTHOUGH its not an "open relationship" IMO if its only the man thats allowed this.