As someone who too enjoys sex scenes in movies and doesn't really get the hate for them - I really dislike (most) porn, and probably for the same reasons.

Sex and intimacy are a huge part of human relationships. Sex scenes in movies show (and not just tell) those relationships, where they stand, how they develop. There is no real intimacy in (most) porn. Porn doesn't tell stories, it doesn't show human relationships, it just depicts body parts smashing into each other basically. Very rarely a porn video is good enough to suspend my disbelief so I can imagine the important, the romantic part of it and forget that I'm watching either the product of an explorative industry or the fetish of a couple who film themselves because they like to have themselves watched by strangers. But to show me the dynamic between two fleshed out characters, how it changes over time, and what relevance sex has to them - porn can't do that.