Lee, it's kind of funny - if I understood your post, and the replies, it seems that several have gotten the situation wrong, and the ones who did are men.

If I'm getting it right, your friends are not cheating on their partners, they are single, and having affairs with men who are in relationships.

Yes, it's more common for "committed" men to have affairs, but they certainly don't have a corner on that market.

The cheater almost inevitably tells the third wheel, "My partner treats me terribly, doesn't appreciate or respect me, we haven't had sex in two years, (s)he's having affairs, it's a loveless relationship, you are more important." Usually a lie, and precisely what the third wheel wants to hear, and they believe it without question.

The third wheel feels so noble, rescuing this poor, unloved cheater from the awful situation. The cheater realizes that the third wheel is providing all the benefits with no obligations or responsibilities. Usually, neither one is willing to face reality. If he had not left his "real" partner before they met, he's not going to, regardless of his claims to the contrary. Period. And even if he did, then the girlfriend becomes the "ball & chain," and he will cheat on her.

Of course he no-shows, breaks dates, ignores birthdays, doesn't call. There are no consequences, and she puts up with it. Perhaps not as physically violent; but psychologically, it's not too different from battered wife syndrome. He dishes it out, because she takes it.