The third thing is: clarity.
Sex can be rough and tumble, with one person on top then the other, with their arms and legs entangled. That’s good when that happens, and sometimes you’re too busy feeling and doing to really keep track.
But as a writer, keeping track is your job. You have to know where your people are, and write it realistically and clearly. For example, if your characters are having rear-entry sex, the man cannot kiss his lover’s eyes. At best he can kiss one of her eyes, but you should mention, first, that she has turned her head. He will have to lean right down to manage it.
If either of them has a tool in their hand, whether it’s a vibrator or a cane, it should be where it must be. If it’s in his hand, and the writer hasn’t mentioned him putting it down, then it’s still in his hand. If he has put it down to stroke his lover, then he has to pick it up again before he can use it.
And the writer has to record that. And so on.
I once read a book in which the hero has sex with the heroine at last. In the morning, the writer told me, he woke up naked, his withered hand resting on his thigh. So I leafed all the way back to the beginning, to see if he had a disfigurement to his hand. It turned out that the writer meant that his cock and balls, resting on his thigh, looked a little like a withered hand. Well, I thought, if you say so.
People often get metaphorical when writing about sex: waves crash, fires light, and trains even go into tunnels. Most of the best known metaphors are dead, really. Overuse has killed them, and they communicate a writer’s lazy boredom rather than sexual intensity.
My preference is for saying what’s happening, in direct language that tells about the state and the action of penises, vulvas and mouths.
A note on metaphors and similes
Only after I’ve done that will I try to think of a good metaphor. Part of a metaphor being a good one is that it shouldn’t have been worked to death by other writers. The other part is that it should make sense, and communicate something specific to the reader.
For example, I once said a masturbating woman’s orgasm noise seemed “high and lonely, like a seagull’s cry.” Almost everyone knows what a seagull cry sounds like, so the simile communicates something about how it sounded, and also something about her emotional state.
Another kind of orgasm could be said to be like the sound a cat makes if someone rides a bike over its tail. That is arguably an accurate simile, for some orgasm sounds, but it’s a bad one because its too outlandish, too far removed from a sexual context.
Metaphorts and similes should be accurate, appropriate to the emotion of the situation, and not too outlandish, or too commonly used.
The fourth thing I try to achieve is… humanity.