I guess since I am not a consumer of any of these, I could be way off base, but I always thought the terms rather meant a range.
Romance - The primary story is about the relationships, and interactions. Sexual activities are just one optional factor in that.
Erotica - The primary story is still mainly about the relationships, but the sexual activity becomes a major player in the plot.
Porn - There may or may not even be a relationship, or much of a story, as the primary focus is on the sexual activities.