I would say this comes into play in long distance relationships. Being away from someone for over 9 month's made me realise that i both truly cared about her and loved her, but also that i had physical needs that jill couldn't take care of. So i told her that, and she basically said the same was true for her. So we agreed to stay emotionally true to each other, talk on a regular basis, and visit each other the few times we could, but also that we could hook up with other people if either one of us wanted to, and neither of us would hold it against the other. We both had sex with other people, and at the end of the day we were able to get back together and be fine.
That being said, i don't think either of us would have even considered cheating if I hadn't moved, or we would have been able to see each other regularly. The need never would have been there. And if she would have fucked just one other guy if i was there, that would be the end. She has said the same is true for me.
I really think that is the only kind of understandable circumstance.