Few will admit this....but
If you go back to someone who cheated on you, you essentially give them a pass to do it again. Now they'll deny it. But it's a psychological thing. They know they won't get the axe for taking it outside of the relationship.
Now I suppose there are people out here who don't go out and act on this pass....there are exceptions to everything...but the person that takes them back? It has to eat at their mind hard. Unless they took to fukking around themselves.
This world is far from perfect.