The day I tried to write my first sex scene was one of the most embarrassing moments of my life. I'd avoided it for a long time, and then I realized that the story I was writing demanded it (not the first time I'd changed what I felt I was capable of due to the demands of a story). I had this idea of what had to happen, and I tried to write it. When I got through I realized it had devolved into a succession of meaningless generic actions and disconnected body parts.
It was awful. And, I realized, it was "sex-driven" in a bad way, the same way that stories can seem pointless and over-wrought when they are too heavily driven by plot.
Something changed for me at that point. I realized that that the point of a sex scene was not the sex.
Why do we need sex scenes? I suppose for erotica that they would be part of the point, but in my stories that's not it at all. In my stories, I have two people developing a relationship, and what is most important is what that relationship means to them, and how it changes them. I had already figured that out for kissing scenes, so that was where I went when I had to re-think the sex scenes.
As I see it, a first kiss is a form of communication between the characters. Tension may be building - and this is something I do by having the characters become more aware of one another physically, say, noticing for the first time the way the other person's throat moves when he drinks - but somebody starts it. The other person then has to decide whether to permit the kiss, and whether to return it. Internalization is critical here. Too little internalization and it will seem like I've slapped the kiss on from my position as author. More internalization may make it seem like the poor character is in agony trying to make the decision (which he or she may be!). Occasionally, since this is a big turning point in a story, I'll switch points of view and place the kiss itself at a chapter break so I can then move into the recipient's head and gauge the reaction.
What is important is not the movements. Yes, we can say "oh, this is how far they went this time." But what is important for me in a kiss is the nature of the communication - the psychological conditions that permit someone to take the chance, and the experience of the other person in response.
A sex scene is the same for me. The question is much less "how far did they go" but "what did they decide to do and why, and how did it affect the way they will interact in the future?"
I therefore place my focus on the characters. I start by asking, "What significance does this scene have for the characters, and for the story as a whole?" That will help me gauge what is necessary. If the scene is incidental, like a scene demonstrating that a character has sex as part of his everyday life and doesn't think much of it, then it will get a lot less attention. You'll see where the couple make their decision, and follow through with little detail, the critical ingredient being what the act means, and what it does for the characters, rather than what they do. I have one scene where a character makes love with his girlfriend because this is something relatively normal that they do often, and it helps him to release anxiety from the earlier part of his day.
The buildup for a first sex act is usually much longer. This I think is natural because, compared to kissing, the first occasion of such intimacy has far greater significance - and much greater possible disasters associated with it. Romance novels, after all, can spend almost the entire book getting there! What I have found, though, is that in this case the physical act itself is far less important. I can build up the psychological conditions necessary, and once the two characters have made the decision to act, I can end the scene. The only reason I might include physical details is if there is some consequence of the act itself that must be experienced in order for readers to understand the characters as they carry forward.
All of this is to say that I recommend including only the most character-relevant details in a story, either when you're dealing with a kissing scene or with a sex scene (or anything else, for that matter!). Keep the motivations, the decisions, the justifications, whatever it is. Keep the mental states that matter in the front of your lens, and let all physical details follow directly from them. It's the best way I have found to create a scene of intimacy that actually fits the characters I'm working with, and matters to the story, without letting things fall into clichéd motions and lists of body parts.
I thought I'd revisit this post because I recently wrote a scene that demanded more than my usual amount of attention to the "sex part." Note that I didn't say "parts" - but this scene was one I had been building up to for a very long time, and it required me to go all the way through the sex for several very specific reasons. The process I'd been going through as I went through the story over the long term was making a mental list of ways that the two people were not compatible or would not consider one another, and then knocking them down one by one through the events of the story. At the point where they became intimate they had to have quite a deep discussion about it - so that was how I covered the "why," but because both characters were important, and both viewed physical intimacy in vastly different ways, how they did what they did became very important. What did each one consider "too normal" to be appropriate in intimacy with the other? What did each one consider frightening? What did they consider not worth noticing (say, whether the lights were on or off) and where did they put special attention? The other reason that I had to carry through was that the fact that they consummated the sex is actually very important to the way they will interact in the future. This is to say that the relevance questions haven't changed, but in some cases the story and relevance questions will demand the entire scene, and sometimes they will not.
It's something to think about.