1. There aren't any.
Seriously? You actually thought I would condone this pathetic, childish, immature, unfaithful behavior? Shame on you.
If you were actually looking for reasons to cheat on your partner, thinking this link would help you, shame on you.
2. Don't be a piece of shit.
Don't do it. Even if you think they deserve it because they cheated on you or YOU THINK they cheated, don't lower yourself to that asinine level. Be mature.
But if YOU are thinking about cheating, or currently are in any way, you're an ass. She/he can do SO much better without you. The best thing to do if you want to mess around with people that don't matter is just to leave. You're already in a different mind set, not caring about your S/O's feelings, so why drag them on? Be mature.
3. Leave them if they cheat.
To those who have been cheated on but chose to stay with them: you're an idiot. I don't care what the circumstances are. If they cheated, you know they are fully capable of doing it again. If anything, they just figured out new ways to get away with it better or longer.
Get out of that shitty relationship.