Cheating is one of the worst things anyone can do in a relationship. You go behind their back and do something you know will hurt them terribly if they found out, so you lie to them, selfishly sneak around behind their back and willingly risk breaking their heart just because one ****buddy isn't good enough for you. Cheating is the lowest of the low. Once a cheater, always a cheater. No second chances in my eyes.
If it's a consensual open relationship, then who cares? Some complications might arise if people change their opinion on the situation, but everyone knows what they're getting themselves into so there's not really any selfish or malicious intentions involved.
If a relationship is falling apart, it doesn't make cheating okay. If someone is in a bad relationship then they should just end it before pursuing another partner, it's not fair to sneak around because you're too cowardly and selfish to be honest with someone you're supposed to love.
If the relationship is such a bad one that you don't love your partner anymore, then there's no point in cheating because you might as well just end that relationship if there's no love left. If you do love someone, then you just wouldn't cheat. I don't believe anyone who cheats on their partner can truly love or care for them.