To add in a new thought, there are some couples who are very happily "open" and treat sex as a fun activity, like they would go skiing. So for those couples, sex with others is not a problem at all. But the difference is that they openly have the arrangement and do not hide anything from each other. It is the lying and breaking of a promise that is the core part of the cheating. To me if a husband was sneaking off every night to be with another woman, and he spent all time snuggled with her but they didn't have sex it wouldn't be "OK".
