I don't have a complete answer, but I can tell you this -- at least in the US, the term "pornography" is not a legal term of any note. The United States Supreme Court has said that legislatures can regulate "obscene" materials, which may include pornographic material. I've not paid all that much attention to federal lately law since my state gives more protection to speech than our federal overlords. But if memory serves, material is obscene if it appeals solely to prurient interest and lacks any scientific, cultural, social, literary or political value.
So pornography may be obscene. Or not. You might find this
interesting – it walks through some of what you’re touching on.
The take away I get from my background in law and the wikipedia article – porn is an area on the entertainment continuum. Erotica is another. Literature is another. The more the goal is just sexual excitement, the more it’s porn. The more the goal is other themes, the more it’s traditional literature. Erotica is in the middle. I’ve never heard of Romantica, but I’m bettin’ it’s randy romance. Where people draw the line varies dramatically.