Cheating is so common that there is a television show called "Cheaters" which helps people confirm if their partner is in fact doing the do with someone else. You see the anger, guilt, tears and lies that cheaters tell even after being caught.
So, why do people in a relationship go astray and cheat? Check out this article I came across, it's pretty interesting and may give you some insight, personally I think a cheater is a selfish, immature individual. What's is so hard about being honest and just telling your partner that you don't want to be in a committed relationship? Stay single and date whomever you like and you don't hurt someone you "claim" to love and care for. Read the article about real reasons for infidelity yourself. It'll make you think.