IMHO, full-on erotica is about sex. If the story is about an unlikely pairing, it starts as close as possible to the first encounter, then deals with its fallout. Romantic erotica - on the other hand - would build up to the encounter, with pages and pages of tension.
However, it doesn't have to be full-on sex in Chapter 1. For example, in my novel (published a couple of years back) the 2nd half of Chapter 1 had the hero play peeping tom while an attractive girl in another building um... well you get the idea.
The scene was part of the plot because it made him realise he had to do something about his plight (stuck in a male chastity belt, so unable to take advantage of what he was seeing), and it set up the girl in question as a future partner.