if your in a situation like the one with sienna miller and jude law...

i don't think i would. Cheating is the worst thing that you can do to your partner, and there's no such thing as a 'mistake' in that case, cuz it's a conscious decision that you make. And mostly people who cheat on all their partners seem to have a pattern and sometimes are just incapable of having am monogamous relationship, in this case if you can't handle it, then you should move on, cuz people don't change.