Yes, I was cheated on about ten years ago by a guy I was living with for two years. He went out and partied with an old friend, and one thing led to another.
He told me the next day they had slept together.
We tried to work things out after, and I forgave him, but it was not to be in the end for different reasons.
I think that if cheating happens, then it is for a reason. Somethings wrong in the relationship.
I actually regret forgiving him because it prolonged the inevitable.