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.
In other words, a way to play out fantasies in a safe environment.
Consider this reaction: Just as casual sex is not necessarily inherently harmful, neither are online affairs.
But they may be so when participants are also involved in another primary offline relationship, because of the harm imposed on those partners.
Nevertheless, since online affairs are real they do often cause actual harm to one's primary, offline romantic relationship.
Accordingly, many people will be just as disturbed about a partner's online sexual affairs as they would be if they discovered that their spouse was exchanging steamy love letters with someone else.
Time spent in that world can help them their actual world, while not giving up on having exciting, even emotional experiences.