Much writing about sex goes wrong because an author suddenly shifts gear and starts describing a physical process.
The point can be made by taking, as an example, another process, not sex. If you write, say, a description of a person becoming depressed on the street and then opening her front door, you might compose something like this:
“She easily extracted the key from its usual place in her handbag and inserted it into the lock.”
Now if you expand your description of unlocking a door, even with some attached memories and feeling, into a five hundred word text, you will probably find yourself writing utter drivel. Quite simply, the well-known mechanics of unlocking a door do not need such an extended description.
The same is true of writing about sex. The mechanics of having sex - even of perverse kinds - is pretty much shared information, and attributing evermore over-flowering metaphors to the sexual excitement is what makes it absurd. The way out of this dilemma is not to make the topic of the paragraphs the sex itself, but instead make the theme of the paragraphs the feelings and thoughts and then use sexual activity to illustrate the point.
In short, don't focus on sexual acts, but use sexual acts demonstrate what you are trying to say.