The sexes are vague and it’s a very important fact in this equation. If it was a man that cheated, perhaps understand that it is in his nature to be with multiple women. And make a judgement on whether you are willing to accept this or not. Bare in mind if you do not accept this with this partner you are likely to experience it with your next partner, whether you know it or not. If it was a women that cheated than understand the risks you are taking staying with her, and how unnatural of a relationship where one women is with more than one man. In this case most likely the best course of action is to go your separate ways, unless she truly repents and you are willing to take that risk. Either way make a decision on your action and throw your full weight behind it. If you choose to leave, leave with every fiber of your being. If you choose to stay the same.