It goes without saying that the psychology of affairs is complex and nuanced based on the people and relationships involved. Personally though I think it boils down to this. Erotic experience gives us a sense of aliveness in a way that few other experiences can. It is a very primal, very human need.
Yet in the majority of long-term relationships there is a natural ebbing away of eroticism over time, as lovers become parents and domestic partners, as responsibilities and life pressures increase. You become family rather than lovers. Novelty, which is a significant component of eroticism, is replaced with familiarity and stability. But the need for eroticism remains, a continuous itch to be scratched.
There is a generally accepted assumption that it is morally correct and perfectly acceptable for us to own and control our partner’s erotic experiences, even if we are no longer providing them with those experiences ourselves. Indeed in most cases we are unable to provide them ourselves since we can no longer offer novelty.
We expect our partners to sacrifice their experience of the erotic for life, to never again feel that sense of aliveness, in order to safeguard our own security. So we can feel safe in the knowledge that if they don’t stray physically they are unlikely to stray emotionally, and therefore our domestic security is assured. When both partners are willing this is a fairly typical sacrifice made in exchange for the benefits and stability of a long-term domestic partnership.
People tend to see any desire they might have for erotic experience outside of their primary relationship as incongruous with what they have been socialised to believe love is, and therefore they assume their desire must be symptomatic of a flaw in the relationship itself, even if actually the relationship is a good one. They pull away emotionally, magnifying to their partner and to themselves any small flaws in the relationship, in order to be able to justify their need for eroticism. This is where ‘the script’ comes in.
Too many people equate feeling a spark of lust, the intensity of sexual chemistry with someone new, to finding a new ‘true love’, which is when this idea of erotic monogamy can be dangerous to marriage. We don’t have just one friend, we don’t wear our one favourite outfit every day for our whole lives or eat our one favourite meal every day. But we apply this exact approach to our erotic relationships and scratch our heads as to why it doesn’t work.