So again, erotica contains elements
of sexuality but it isn't all about sex.
Like Thomas points out, "obscene" and "vulgar" mean different things to different people.
Personally, I don't find mainstream porn obscene or vulgar but stupid, sad, creepy, and boring, certainly not erotic. I'm not really opposed to it's production but I don't get the attraction. (I've never seen any of the fetish stuff nor do I intend to so I can't even speak to that other than if it involves children it should be illegal.)
According to the dictionary pornography and erotica are essentially the same thing -- pictures or words used to incite sexual arousal -- but it seems we all agree that there is a difference.