Really depends on what type of relationship they have. In my monogamy views and lifestyle, if I found out that my boyfriend was cheating on me, I'd make sure to give him a good reason to spend a few weeks in the hospital...and all the chicks too.
Is it possible that the person who cheats might not be getting what they want out of a relationship? If so, they should break up with the person of breaking his or her contract. Sex of each party aside.
I don't think highly of someone who risks passing STDs to their partner by cheating on them with other people. Their actions show how little they care for their partner. It's really an awful thing to do. If the relationship is an "open" one than that's a different matter.
I agree with so many people on here; If you are in a relationship with somebody and they cheat its just not okay. There are many reasons a person does it normally to do with feelings they have or insecurities they have about themselves. Fear? Of being committed, or just like the thrill of what they cant have and shouldnt do whilst in a relationship with another >.> People can be cruel sometimes, and it can be quite heartbreaking. Out of experience; If being cheated on, do break up with them. Most people who try work through it, end up being hurt again :/