Hate to break it to you but cheating is far more common than anyone wants to admit. You probably know many people who cheated, or perhaps are currently cheating, on their spouse, and there is no reason you would be aware of it (it is not exactly something people share with their friends or family). If you are going to take the position that someone who has an affair cannot be trusted in any other context you are going to live a very paranoid and unproductive life.