If a partner is a good parent, emotionally supportive, does their bit around the house, etc, I can't really get too upset about them occasionally getting a bit of outside action, providing of course they use condoms and aren't inconveniencing me/kids or over-spending.
Interestingly simplistic. It is actually the lies, deceit and change of relationship dynamic which an affair causes, that kills it. Ime. Everyone is entitled to enter a relationship type of their choosing and everyone deserves respect and honesty within that relationship, whatever the type.
There is a fog but as a pp said, it is a fantasy. They live a fantasy and let reality slide. The fantasy becomes the more important aspect and reality becomes a dream. Both fantasy and dissociation, then?
However, each affair dynamic is different. In my experiences, one entered a fog, while the other did it out of sheer spite. Both (foggily?) thought they'd 'get away with it' and that I'd 'take them back'. This must have been fog induced thinking because they each (in reality) knew full well, that I wouldn't continue a relationship with someone like them. I'd almost believe they were 'get out' manoeuvres, if they hadn't both been ridiculously intent on staying with me.