for the wayward spouse: its the ability to ignore years of a loving marriage/friendship/partnership and compartmentalize their despicable behavior. their heart goes pitter-patter and the adrenaline starts pumping when they are sneaking around. This excitement outweighs any sort of conscience and guilt associated with their behavior (let alone the potential life changing consequences).
for the betrayed spouse: its "he/she'd never do that, we have too much history". Rationalizing their behavior, believing all their b.s., falling victim to gaslighting, rugsweeping, etc. etc.
instead of "head in the sand", its "head in the fog"
Think of when you first started dating someone you fell in love with. Think of how excited you were to see them and how hurt you were when you didn't. You were infatuated with them. Your body was sending a huge amount of hormones to your brain that has the equivalent of a small dose of cocaine or ecstasy.
The "wayward spouse" is feeling that. What I was trying to say in the other thread is if your husband is feeling like this, almost no amount of arguing or proving yourself right is going to make a difference. It would be similar to trying to rehabilitate a cocaine addict. The body and brain in a literal sense becomes addicted to the source of the good feelings they get.
I've never had to deal with the fog before, so I can't really give you advice on how to handle it if your H is foggy. That would be an excellent question to ask on here, though.