I am sad to say that I have once cheated on a girlfriend of mine, and before the event had happened I had always told myself that it was something I would never do. Of course, controlling our emotions is a difficult task, especially in our tumultuous high school years, but, speaking for myself at least, after having hurt some one in such a way, I feel its safe to say it is something I will not do again. It was a temporary lapse in judgement, and it lead to the collapse of both relationships, but having learned from the whole ordeal, I would not say it is fair to judge someone or to potentially not date someone due to them having cheated before. People mature and they learn, and I feel that someone who is wiling to own up to their past failings is at least trustworthy enough for the chance to prove themselves.
Running on almost 3 years with my senior year sweetheart, never came close to cheating or betraying her, the past does not dictate the future.