A combination of the above, I think.
If you look at what the Catechism says about it, pornography essentially violates of the inherent privacy/intimacy of the sexual act (by displaying it to others) and invokes “base pleasure”, i.e. lust. These two characteristics distinguish it from art involving nudity, etc., which isn’t itself pornographic unless used in a pornographic (if that’s the right word) context/presentation.
The first reason wouldn’t apply to violent media by its very nature, but I think a very strong argument could be made that something like the second - their being made to invoke a disordered pleasure - might mean that certain *very *extreme violent movies with little other content might be wrong to watch. (By which I don’t mean ‘ordinary’ movies with violence - war movies, or fantasy movies with big battles like the Lord of the Rings, or monster movies, etc. - but the super gory movies where people get graphically dismembered for lots of screen time, where there’s little story and the movie pretty much *exists *just to show extremely graphic violence.) In more normal/moderate presentations, a lot depends on context & the viewer’s attitude, etc.
Also, yes, people (as a general rule) are more likely to be aroused sexually by sexual images than incited to violence by violent images. (Though this is variable by person - and, of course, people who think they will be influenced by violence in media should try to avoid it, just as someone who’s susceptible probably shouldn’t look at non-pornographic material containing nudity or the like.)
