It would be foolish not to take the circumstances into consideration. Was the person who cheated on you someone with whom you were in a long-term relationship and the cheating was a one-night stand under the influence of alcohol? That would be much different than if it was someone you'd only just started dating and the cheating occurred multiple times. Is the person who cheated apologetic? What are the risks of catching an STD?
There are times when you should never give a cheater a second chance, but there also are exceptions. So I can't give you any hard and fast rules, but if you're really confused about a situation, then you should consult a counselor.