I think it is totally dependent on the relationship. If someone cheats there has to be a reason that the person cheated. Maybe the relationship has been on some seriously rocky slopes, maybe the trust has disappeared, maybe the respect has disintegrated. While some may blame intoxication, there is a deeper reason; the alcohol just amplifies the reasoning and decreases the judgement of the individual.
If I were cheated on, I would do my best to forgive and forget. It would be extremely hard and it would take time to rebuild the trust platform, but I believe I would be able to overcome it with my significant other. If it happened a second time, then I would definitely be out the door.