That's a question which has really ever been philosophical to me. Write what you want. Read what you like. That's pretty much how I feel. Lately, that is, the last year or so, it's become less than academic, mostly due to Amazon. Now Amazon's very vague guidelines state that no pornography can be sold. That's generally been interpreted to mean 'visual' material, but it could certainly cover written, as well. And since I'm sometimes unsure why Amazon blocks one or another of my books it's become of some concern to me.
One shorthand way I've used to decide which is which is simple quality. If the novel is written with a degree of ability with regard to plot, characters, dialogue, not to mention basic grammar, it likely falls into the erotica box. Of course, assessing how much is enough is, at best, a personal judgement. The other day, though, the difference was made stark to me. I came upon a web site which had posted entire copies of 80s era porn novels for people to read. Naturally, I had a look, mostly to see if any of mine were there. I can't say I skimmed it all, just a dozen or so novels, and quickly, but the difference between them and what I write is pretty noticeable.
Basically, porn novels are like porn videos. You don't know much about the characters, and they don't ever bother to explain much. There's not much in the way of a plot, and pretty dumb dialogue. It's basic sex, sex, sex from just about the first page. In the BDSM novels I looked at, the action was cruel, crude, and almost completely non-consensual. Some of it was pretty unbelievable, too. I don't think, even when I was writing 80s porn, that I was ever quite that, for want of a better term, crude. And, of course, every year I move further away from the stuff I wrote back then.
So looking at it did at least settle for me, the question of whether what I write is porn or erotica. Don't know if that point could ever be made to Amazon, though.