I've been involved in a so called "open relationship", except I was the only one who knew it.
So it's more like a "half-open" relationship.
Joking aside, I've been with a girl who did threesomes with me a few times and she had no interest in fucking other guys. If she had wanted to, I may have had to think about it. But then I think about how rare it is to find a situation where you can fuck a girl and some of her friends. And one time it was a stranger. It would be an ok trade off to be OK with letting a guy fuck her.
Threesomes are great. I love to wrap my arms around two girls at the same time. That's the best feeling in the world to me.
We tried to engineer a foursome and it wasn't so easy. There was an exponential difficulty increase going from two girls to three. Eventually I paid three Montreal escorts for a foursome. Best $1500 bucks I ever spent.