Our first duty as writers is to write the truth. That does not mean that we cannot, or should not write fiction or fantasy. It does, however, demand that we not tell our readers things which are physically, scientifically, and demonstrably false.
We can write about things that do not exist, and indeed cannot exist, in our known universe. We can 'invent' truths that can exist only in our stories. However, when we do so, we must also create a consistent and believable logic and a cause and effect rationale that enables a reader to accept the story world and its occupants and the things that happen in that story world as possible and even probable in the environment, and under the circumstances we show them through our writing.
However, writing science fiction, or science fantasy, or even stories of magic is not what I want to address in this essay. I want to discuss writing erotica.
Erotica is that genera of literature which tells us about sex and peoples involvement in sexual activity. It may be quite explicit and include 'gutter language,' but need not do so. It should, however, entrain a reader's imagination and cause him or her to vicariously experience the emotions and physical sensations experienced by the characters in the story. Some of the scenes in an erotic story, if pulled from the story and read out of the context of the story, might be pornographic. That is, such scenes portray the sexual activity in explicit terms purely to arouse the reader sexually and to cause a genital response -- tumescence, and the flow of fluid accompanied by a desire for orgasm
That is good. Those torrid sex scenes are to erotica what seasoning and spices are to food. They are what makes an otherwise bland, account of the interaction between two or more people come alive and make the reader wish that either he were a fly on the wall seeing forbidden aspects of someone else's life. Or wish that he was one of the participants.
However, and it is a huge however, too many such scenes, with no story line to put them into a believable context are as undesirable as what too much strong flavored seasoning is in a dish of food which if properly prepared is delicious and nutritious, and which leaves one wanting to eat more of the same at another meal.