In cases other than where physical or emotional abuse are prevalent, I believe people cheat because they lack the courage to have, or are incapable of having an open and honest dialogue with their partner.
Let's take a look at the top three reasons
stated for cheating...
1. Emotional Needs Not Being Met
... Talk to your partner. Let them know how you feel, how your needs are not being met.
2. Sexual dissatisfaction
... Talk to your partner. Let them know how you feel, how your sexual needs aren't being met.
3. Hurt by partner
...Talk to your partner. Let them know how you feel, what they do to make you feel hurt.
If talking doesn't work...if your partner is unwilling to work on this with you, have the guts to end the relationship AND THEN move on.
Talking about this subject with my sister, she said "stuff happens...we're only human". We can't judge people for their mistakes. While I agree cheating doesn't mean you're a bad person, I believe that if you mess up and make a mistake...don't let it go on and admit to you and your partner that you used bad judgement. Then accept the consequences.
If your partner is willing to work things out, great. If not, so be it.