It is possible to stay loyal despite physical infidelity. People can change and overcome difficulties. It is very natural to cheat, and people are very understanding. Someone might be tempted to cheat in a time of passion, although they are still committed to their original relationship and they do not want to lose it
A person's infidelity means that they've renegged on their loyalty to another person. Therefore, it's impossible to cheat on someone and then remain loyal to them in any way. Too many people think the opposite and wind up paying the price for it with repeated betrayals in the same manner.