There’s nothing saying cheating is good in the this. And honestly yeah, cheating is kinda weird to be on this, but I think it’s just a statement saying “this is a type of relationship, a type with one person cheating on the other.” It’s as neutral towards cheating as it is towards anything else.

If we want to nitpick, “open relationship” uses the word “affair” which has a lot of negative context, and so open relationships must be bad right? I don’t see this as endorsing anything.