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The private nature of online affairs may make them less painful for the betrayed partner as well.
Moreover, when online affairs are revealed to the significant other, which is done more often than when offline circumstances are involved, it could be considered as something less than cheating.
They are also perceived to involve a lesser degree of betrayal, as they involve more imaginary elements and the degree of neglecting the partner's interests may be lesser.
These people believe that if they do not even know the real name of their cybermate—and never actually see them—their affair cannot be regarded as from a moral point of view; it's no different from reading a novel or other form of entertainment.