I am a swinger and there are definitely men like this around. Mostly, they are very together and lovely guys (although of course I don't have to live with them!) and they are almost all single. The only married one I know of is a female swinging friend's husband and he's a typical middle aged bloke otherwise. I think he just can't be arsed with her high drive (or the mental load of their house and kids, actually). I've never met him.
For some reason, men seem to THINK they love this idea and then it causes massive problems when a relationship is opened up in any way. I met my current partner through swinging. He's been part of it before and was wildly excited about me being with another guy. When confronted with it in reality he was absolutely devastated. Our relationship almost didn't survive it. While it's very common to come across essentially single guys who are happy to have other men involved with them and a good friend or low key girlfriend, it is rare in my experience to come across a man in a committed relationship who is OK with it. And men who enjoy it with good friends often openly admit they don't enjoy it if they do meet someone they have a deeper connection with. My theory is that their masculinity is just too fragile and the 'better' the man, the fewer refuges their masculinity has nowadays. I think they are all secretly hoping to compare favourably and that you won't want anyone else after all.
In every successful scenario I have personally come across it is the WOMAN'S desires that drive swinging. Anything I have ever been involved with where a couple is motivated by a man's desire or fantasy has bombed one way or another. They just don't seem to be able to handle the possibility that they aren't the only great option. They don't feel special enough as a man if you can have great sex, right now with another man. Too used to being overvalued, perhaps?
I've seen my partner with a few other women. We have another woman or couple join us once every few months or so. Mostly I feel pride and desire when I watch him with other women and I enjoy it immensely - more than he does, I would say! I'm not jealous - that pleasure is mine any time I like. Her pleasure cannot diminish mine in quality or quantity and it's fun to share with a friend. It doesn't have increased value to me because it's exclusive (although if I did want that he absolutely would commit to it). It has increased value because it's shared. Like a good meal. It would be like being upset that he liked eating in a restaurant occasionally and feeling like he didn't value my cooking (which I pride myself on) enough not to.
Either I'm getting what I want out of relationship with him or I'm not. And as I don't want 100% of his time and energy, what he does with himself or anyone else when my needs ARE fully met is irrelevant. If I'm not happy with his input, it makes no difference to me if he's spending that time and energy wallpapering his Aunts living room, following Puddlemere United or banging his NDN. The point would be that he didn't prioritise me and my needs (not just sex) are going unmet. So he either re-priortises or I find myself a man who will. He has my complete blessing to sleep with whomever he wishes.
My boundaries are that he tells me about it in advance, I know roughly what's happening, and that each person is identifiable to me in some way. I enjoy hearing about it in the same way I enjoy hearing about anything that makes him happy. But I don't need to know in detail if he doesn't want to tell me. He rarely sees others alone, and has much more fun doing all the ground work for someone we see together (which I find a bit tedious). I think he enjoys the possibility more than the reality. I would never ask him to do it if he didn't independently want to.
His boundaries are that he wants to know if I meet other men. I occasionally have and while I know he doesn't like it, he doesn't treat me any differently. It's rare I do because he's the best lover or partner I've ever had, and he gives me everything I want in a man. We don't live together (because I don't want that) if that makes any difference. We both have teenage children and I don't want to blend families. We may in future, but I really like things the way they are. I know it isn't quite the same, but it perhaps gives a bit of insight. I'm happy to answer questions.