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"