My opinion: Cheating is more acceptable than swinging for 1 major reason-People like to feel like a victim. Swingers (for the most part) have accepted certain truths about themselves and have thrown off the sex/guilt paradigm. A non-swinger/open-minded couple would rather have the moral cover than admit that they're incapable/unable/unwilling to meet their partner's needs.
Case 1: man cheats on wife.
Wife: "He fucked some bitch!"
Friends: "That bastard!"
Unfaithfulness makes her the victim of a lecherous man.
Husband: "She hasn't let me do anything besides kiss her cheek in three years. Except for a 5 minute anniversary fuck. And if I don't come, then that's it for me!"
Friends: "Shit, that's fucked up. Once a year? What did she expect you to do, just dry up or explode!?"
Unfaithfulness due to lack of consortium makes him the victim of a frigid wife.
Case 2: wife cheats on husband
Husband: "I can't believe that she'd do that! I do everything for her, work all of those damned 12+ hour days so she doesn't have to do anything!"
Friends: "She's a whore, bud!"
Wife: "He's always too tired to do anything, and his stamina has dropped as he got older. And it's just in and out, nothing else, ever!"
Friends: "How inconsiderate. Just taking his own pleasure and leaving you frustrated. What a bastard, treating you like a blow-up doll!"
Sound familiar? No matter how "heinous" the behavior of either party, that person can always convince their friends (old and new) that they're the victim. And by being a victim, they can assuage the society-conditioned guilt of not being faithful. It's not about being faithful, it's about keeping up appearances.
By embracing the open relationship/swinging lifestyle, a couple is breaking that chain. They're saying that society is wrong, that it's needs and requirements for harmony aren't sufficient for their needs, and that they can do better. And for a lot of people, bucking standard trends is a major no-no. An "arrangement" may be accepted (he says nothing about having 4 brunette children and 1 redhead, or she ignores a random make-up smear on his shirt if his trip lasted longer than 2 weeks), but open acceptance is intolerable.