Imprinting.
I’d venture that people who marry their first sexual partners are imprinted on them and more apt to have long happy marriages.
I’d further venture that having multiple partners blurs the imprinting and starts an imprint on variety, rendering a single partner solution incomplete and unsatisfying.
Yes on all points.
I strongly agree with your first comment, but on the second one I have a different ‘view’.
You could call it “a loosening of your morals” and once you’ve done something, it’s easier to repeat. Once you steal, once you lie, once you cheat, etc. Broken Commandments.
Those who watch porn become bored and crave more and ‘different’ types of porn. Nothing good can come of it.