What Makes it Erotica?

I've been chatting with authors recently about labels. Genre labels.

Why are some mainstream books that contain lots of sex not labelled erotica?

Why do I label my writing and art erotic? Or at the very least, sensual?

Well, I guess I can only speak from my point of view. I worry sometimes that labelling a piece of work 'erotic' is putting a warning on it. Why would you need to be warned about a story that contains all facets of human existence? Birth, violence, sex, death, longing, desire - all...