Hang on a mo, do you think all non-monogamous sex is on a par with adultery? Because it doesn't seem like that to me.
How about a hypothetical example. L and R are a long term couple. K has two girlfriends J1 and J2 both fairly long-term, J2 has another boyfriend P who she's also been seeing for some time. If R and J1 and J2 are all happy about it, and P has no real opinion, is there any reason at all why K and L shouldn't go to bed together if that happens to seem like a good idea to them? Would it be different if any of the pairs involved are married? Is any of it adultery?
I don't mind if Richard wants to sleep with other people, I'd rather he checked if I have any specific objection to an individual before going ahead, but it's unlikely I would. I'd expect him to tell them the situation (if they don't already know) and to take sensible precautions.
Just because a relationship is non-monogamous doesn't mean there's no concept of cheating or lying. And doesn't mean it's not something to which you can have a lifetime commitment. I intend to be with Richard for the rest of my life.
This doesn't stop me being sweet on several other people. Some of these I won't do anything about because they're seeing someone else and are monogamous, and I'm very happy for them in their current relationship. Others because they believe in monogamy and aren't happy with seeing me under these terms even though they themselves are currently single. Hell at the moment I'm barely interested in sex at all (side effect of pills).
I'm pretty sure I could be monogamous if that was what Richard wanted. I've occasionally slipped up in monogamous relationships in the past, but I think I learned from them. Sometimes it feels life would be much easier that way even. But that's not the way it is.