I think we imprint on a particular core fantasy when we are maybe 10, 11, 12, whatever.
Then that's what does it for us in our sex life. That particular core fantasy is what gets us off more than anything else. It's a fantasy, yes, but it's also based in reality in some way. It isn't so off base because it originated in the world of living breathing molecular boys and girls.
In the "old" days it would be an experience such as: Going to a party, a girl lets a guy feel her breasts, for the first time. And maybe he got very excited, maybe he comes, maybe not. But he uses this for masturbation fantasy purposes afterwards and it imprints. Now, forever more, he gets eternally turned on from breasts that way. Et cetera. Yes, the guy might appreciate a nice ass, but that breast thing will do it for him for the rest of his life.
I think porn makes our core fantasy go whacko. Instead of settling in and living our life with that core fantasy, we chase after one or another, usually more intense than the last, and that is why we can never get real life to measure up.