I ended the relationship.
Cheating generally entails lying directly or by omission. When you can't trust someone the relationship is pretty much over.
I suspect the main reason why some people are quick to "forgive" is because they are so hurt from the knowledge of being cheated on that the very thought of a breakup or divorce is too much for them to digest.
There are some people who see breaking up as letting the "other man" or "other woman" win. Therefore they're determined to "fight for" their relationship/marriage.
"Never love anyone who treats you like you're ordinary."
- Oscar Wilde
The reality is no one can stop someone from cheating. Monogamy is an (individual's choice). Assuming one knows how you feel about cheating and they went ahead and did it anyway should tell you just how much they care about you!