That is a very interesting question, which deserves and answer. But the truth is that if you watch discovery health they were doing a study on it and they come to find out that the reason why people cheat is because of a chemical in their brain. It keeps them wanting more, it keeps them from staying with what they got because they get bored. Not everyone has this problem but it's just like a drug, once your addicted your stuck on it, that's why we have sex addicts. And the reason why they think it's not a big deal is because they don't know what they are doing is wrong, and sometimes even when they do know what they did was wrong their brain tells them that they need more and such. Sometimes it's not their fault, but most of the time it is. No one should cheat on someone, if you don't want to be with that person than leave them first. It saves a lot of people a lot of problems, and it will save you from hurting someone.