The more porn you consume, the more desensitized you become to it. So people turn to more extreme fetish content. It's weird.
This is your answer, OP.
Eventually the dopamine doesn't hit as hard and the softer stuff doesn't make you cum as much, so you seek even more extreme shit. Same can happen in marriages - when vanilla sex gets stale with their partner, they'll usually suggest ways to "freshen up" things in the bedroom and do weird shit.
Sometimes this leads to cheating, too, and is often the excuse given as to why people cheat. Either one spouse no longer wants to have sex because of children, responsibilities, work, etc.; and sometimes one or both of the people in the relationship are tired of sex with the same person over and over again.
I'm absolutely not excusing cheating, but this is a somewhat common or uncommon occurrence, and is very often why people cheat in relationships - because they're simply bored. Though some people are just natural sluts/sleazes and will cheat no matter what. No point trying to pursue those kinds of losers.
