No way that it is that extreme. The ignorance on display here is spellbinding (not only in your post, but across the board). It is much more likely that the person, or relationship is missing something.
Most people don't even want to cheat (certainly not sexually), they find themselves being listened to, appreciated or valued unlike they are in their own relationship. They find that they have emotionally crossed the line AND THEN sexually, which is the opposite of how it happens in 'conventional' relationships.
The victim/guilty party dichotomy just doesn't apply anymore. Realtionships are complex and affairs even more so given as they involve a third party and secret lives, there really needs to be a new understanding of the paradigm around cheating.
Also, attributing it to mental illness, is reductive and insulting to those who have a mental illness and to those who have been involved in affairs.