Implicitly, the root theme of all erotica is the human need to touch and be touched. We are primates.
Acts of sex are central to every erotica narrative (even in their absence), but they are not erotica's primary subject.
The primary subject is the
how
and the
why
of sexual behavior. The
what
remains inanimate without them.
To remain dynamic, the
what
needs to have a narrative function. Sexuality becomes an agent of transformation. The emotional entanglements, the multiple consequences, the ecstasy and the anguish of a sexual relationship substantially alter something within the main character. Loss and gain. For better or worse, life can't be lived as it was before. Sexual intimacy has shifted everything.
Begin the story with its first sentence, then never take a backward step.
Each successive sentence adds something that is immediately relevant to the narrative and previously unknown to the reader.
Invent continuously. Aim for the unexpected. Chopin!
If the story can appear to be continuously invented by the writer, it can be continuously discovered by the reader.
No prologues. Begin with a hook, a moment that leads inevitably to further action. Dramatize it, don't tell it.
Keep exposition short and scarce. Make the story come out in dialogue, action and setting.
Effective writing about sex cannot remain literal for long. Literal description leads to a whole lot of telling with no showing. It becomes exposition and reads like a punch list.
Effective writing is imagistic / aural, figurative. It works the senses. It invokes sense memory.