Not saying it is not fun, just saying it gets boring quickly. Especially if it is always with the same person. Why do you think people cheat each other? They do it even when they love each other on a personal level.
It is just our instincts. We are two legged dogs. We want to fuck everything that moves.
Simple solution: Negotiate an open relationship. Then you can fuck all sorts of people and you don't have to go to Barcelona because your wife does. She can go with a boyfriend and you can do the Colorado mountains with your woman.
All works out.
But it takes the fun out of it when you do it like that. People are also addicted to the adrenaline. (You know doing non ethical stuff in secret with the fears of getting caught)
If I was going to make it open before starting a relationship, I better not start it in the first place.
*btw I am single. I am free to do these stuff. Don't make it personal. I am talking in general. These stuff happen every day. Don't tell me every marriage in the world is honest. Most of them isn't.