people make mistakes, people sometimes do stupid things, but an entire relationship shouldn't be thrown in the trash, for an indiscretion. If your partner was doing it all the time then maybe they are just not qualified to be in a committed relationship
There has to be a problem somewhere in the relationship, for someone to cheat I think. Although the other person may not even be aware of the problem and not given a fair shot at repairing the relationship. Which makes the person cheating, even more of a d-bag for not trying.
Assuming this is related to some of your other questions - it's only cheating when you are betraying trust. If you are in a consensual relationship with someone, and you both agree that it's ok to see other people, then it's not cheating.