Erotic literature is about the erotic relationship between two people. In most cases, this would include the sexual relationship between two or more people. Erotic readers pick this kind of literature because they expect a sex scene. Chances are they expect more than one, but Sex Scenes can be awkward for authors to create. It's one thing to have sex with a partner that you love, but putting the words on a page and describing a sex scene can be awkward, embarrassing, and possibly even one of the worst experiences you will have as an author if you're not familiar with the genre.
It's important to know that the sexual experiences of the characters are just as important as the storyline. Readers expect you, the author, to think about the sex scenes as they have to make sense for the story you're trying to tell. It's simply not enough to just add a sex scene for the sake of adding a scene. You need to think about what your characters are doing and why they're doing it.
Readers can be judgmental
For some people, sex is not a big deal. Plenty of people have one-night stands where their partner isn't really significant. Thus, a one-night stand can mean the two people have sex and then they never talk to each other again. For others, sex is a much more intimate act and there needs to be love and meaning behind it.
It doesn't matter what your storyline is: your sex scene has to make sense.
If you are writing a story about people having a one-night stand, make sure that your characters have a reason for having sex. Maybe they haven't had sex in a long time and they need to have a one-night stand to scratch the itch. Maybe your characters have a one-night stand because they're stressed out at work and they don't know how to relieve the stress.
It doesn't matter why your characters are having sex but they do need to have a reason.
Readers are very judgmental. If you are an author who simply writes sex scenes for the sake of writing sex scenes, they will call you out. Your readers probably know what sex feels like and they probably know how sex happens so they understand that part.
What they don't understand is why your characters are having sex. That's why they're reading the book.
The storyline helps set the scene
In Erotica, the plot and the characters matter just as much as the sex scenes. Two or more people can't just have sex -- there has to be a deeper purpose. Going back to the one-night stand example, stress at work, or a need to scratch the itch after years of being single -- these may be valid reasons as to why your characters are hooking up. Be critical of yourself when you're writing such a scene, as it has to make sense. If you are using it as a filler, then scrap it.
What do you think about adding these kinds of scenes in your writing? Are you surprised that sex scenes aren't necessarily important in your book if your characters aren't adding anything to them?