First — I will attempt to keep this on a professional level of discussion and not delve into a lot sexual words.
I was asked why I don't put sex into my stories. The answer? Simple. Most of my stories don't have sex because they are sexless tales where sex is not a character within the story. Yes, sex is a character. Why? Just like your lead character, sex needs to be touchable, realistic and believable.
To toss in a what is called the "gratuitous sex scene" is really to demean your tale. If the story doesn't need sex, why add it? One doesn't toss in a pirate just because the story takes place in place in the Caribbean.
Yes, if your characters are frolicking on the beach, kissing in the wet surf, the potential for a sex scene is plausible and it should occur. While a couple horseback riding in the surf more than likely won't have sex. It is the moment and the writer should know what is going to happen next.
My writer's critique group reviewed my one novel and one of the members was upset by my continual "teasing" during the story. She claimed she was expecting an intimate moment and I would write something to preclude that possibility and she found it extremely frustrating. Subconsciously, I never realized I was doing that until it was brought to my attention. I addressed the situation with my characters sneaking away for a tryst. Did I write a sex scene. Of course not! For the most part, I write what I like to call sexless tales. Although, in all honesty, not all of them are sexless, but I use more build-up and afterglow than actual details of the act.
My co-author in the novel, Ancient Blood: The Amazon, took a scene I'd written which alluded to sex and took it a step farther. At first, to be honest, I was prudish and wanted to write it out but decided the genre could handle the sex. I did have to reel him in a couple of times when he got a little heavy-handed in the sex scenes but there were some which needed to be explicit and I had to bite the bullet. It was a simple case of either taking it out and making the story totally void of sex or leave it in and live with a blush.
So how does one write a sex scene?
Remember, the first time is much more different than the third time. Be sure to take that into account. Feelings of each individual will be different.
To see if your sex scene is real, I highly suggest you use one of my edit tricks. Write the scene. Go back and read the scene aloud — slowly, enunciating each word. Does it sound real or awkward? Is it plausible or comical?
Now, if you really want to get into the nitty-gritty of writing a sex scene that makes your reader sweat — that which I call "porn" or "smut" — know what you're doing. What I mean is simple — read books written by others in the erotica field and see how the scenes are assembled. Tear the scenes apart and really study them. Learn how they use the terms and depict the act. In other words: Learn the art.
Writing a sex scene is about taking your reader by the hand and guiding them along the sexual route you've created. You will lose them quickly if it is a clinical path where human anatomy used medical terms. Of course, if you use "nicknames" it could become a comedy of errors for the anatomy and action. In other words, if it barks like a dog, pants like a dog and runs like a dog — it is a dog. It is not a doggie, a woofie, nor is it a canine. Point made?