Have you ever seen a picture of a hot chick, only to look down in the comments and see: "Man I'd love to see her get plowed by a BBC" (or some similar variant). Why does that happen? How does someone get to that point?
The answer is of course is excessive masturbation and pornography. What's happening is that someone has shunned real sexual experience for a virtual sexual experience. Instead of participating, they observe another man fuck the woman they're interested in. This plays tricks on your brain.
The porn star essentially becomes an avatar for the viewer, through which they can vicariously release their sexual energy. If it gets bad enough and habitual enough, masturbation to porn may be the only sexual outlet for that person.
Naturally, "I want to fuck her" transforms into "I want to watch someone fuck her" because watching someone else fuck is the only way these people know how to experience sexual gratification. You have essentially trained your brain to equate sex with "watching another dude plow the chick I want to plow".
I'm not a crazy no-fapper or anything like that, but the mind is a fickle thing, and porn is a dangerous rabbit hole. Be careful out there.