This is K (the male)
It is definately cheating and any couple or single that meets someone who is cheating is definately setting themselves up to be dragged into a mess. I can attest first hand to how bad it hurts when you find the other is cheating on you. My ex wife did that and that is why we are "ex's". believe me it is almost like your spouse died. It hurts that bad for the one left out of the loop and the trust is almost impossible to ever rebuild. I just don't want to ever be the cause of that kind of hurt to anyone.
Remember what goes around comes around. That is why we screen so hard to make sure we don't end up with a cheater!
If they really want to be swingers they need to let their partner know and if the partner does not want to participate or give them their blessing to go it alone then they need to get divorced or forget the whole thing.