There's more to erotica than just writing about people having sex, you need to build tension.
I would add that this is a bare minimum. While things like story, characters, and realism are nice, at the end of the day, you have to arouse the reader.
I'd say there's more to erotica than Wish Fulfillment. At least to me, this is a major turnoff. I'm weird, some people seem to actually like writing a literal omnipotent God.
I don't get anything, personally out of a premise like someone finds a genie, and wishes for beautiful women. Or someone discovers the perfect aphrodesiac so women find him irresistable, and can't refuse him. Or hypnotizes them so they're under his power, and can't refuse him. Or he asserts his Power, forces them, and they orgasm like they never have before, because of it.
Especially when there's no real effort on his part. For example, the guy just learned hypnosis. Like he snaps his fingers, and just like that, he can enthrall women sexually. Or he has that perfect body from the beginning. (Pretty much like she's blond, and blue 5'2" and 36/24/36D) Without even mentioning that he works for it, or owns a set of weights. It's just there.
This is basically Lazy Writing. Essentially, he gets "Sex" (Actually rape) just like that, with no effort. He doesn't have to listen to her. He doesn't have to learn anything about her (Other than her bra size) the female character is basically a pair of tits, and a pussy. The man is a 10" dick, and because he has that, gets whatever he wants.
To me, personally, that's about as sexy as a guy goes to the sex shop, and buys a blow-up doll. However, those sorts of stories seem extremely popular. So, what do I know?