I've pondered the odd business of writing sex before, but a good post by US writer Sebstien de Castell, about writing fight scenes, made me start thinking about it again. Sex and violence are hard (that's only the first double entendre) to write because both kinds of arousal involve an altered mental and emotional state which interacts with relatively complex choreography; what happens isn't built of words even if words are involved. Fiction has the same complexity, because it tells stories through characters-in-action as drama does, but in nothing but words. It evolved so richly because it can evoke and depict consciousness as part of the action, and so there's always a tension between those two different jobs.
But that makes sex scenes an irrestistible challenge to many writers, while others will do anything to avoid putting them in. The extra complication is that since humans seek out altered mental states, the pornography industry has thrown up large Scylla of crude words and cruder outcomes on which we might wreck our story, while there's an equally large Charybdis of engorged and purple prose on the other side. The channel between is very narrow, so how to you steer a safe course between them? De Castell's points about writing violence, translated, are a good place to start:
Every sex scene should advance the story. If the sex is just part of their daily (or yearly) lives, then the actual playing-out of the scene doesn't do anything to get the story further. Mind you, if this really is once-a-year then it is probably hugely significant, and that's the point: the only reason for including a sex scene is that it's a point of change for at least one of the characters. You always need to be asking the question: "What is different for them, between the beginning, and the end of the scene?" If the answer is "nothing", then don't write it.
Sex is about character-in-action. How people act is controlled by who they are, and who they are is revealed by how they act, and that shapes both plot and story. What needs are they trying to get met at this moment? And how does that fit in the drive of the story as a whole? The latter often gets neglected: it's easy enough to make us believe that X wants go to bed with Y as a piece of plot (a turn on "the route you take"), but how does that fit in the larger story of "the journey you make"? Does the sex promote, divert or thwart what we thought the route and journey were at the beginning? If it does none of these then, again, don't Show it, even if you Tell us that it happened.
Sex is dialogue. It's a dialogue between two minds and two bodies: hoping, listening, tempting, enjoying, adjusting, mistaking, refusing, ignoring, suppressing. (Even - yes - when someone's on their own, it's still a dialogue between mind and body. A Secret Alchemy starts with one of those...) If you get stuck, it's always instructive to sketch out the same scene from the other person's point of view. If you're working with a moving point of view, where might it change?
Fulfil the promise of the book: this is about staying true to the tone and style of the story. If it's light-hearted, don't lose your sense of humour just because your ferociously feminist mother-in-law will disapprove of the silly words. If the story is all gritty realism, don't wimp out just because you're reluctant to be as grim about sex as you have been about drugs and rock 'n roll and the nightmare which is the self-checkout in your local Tesco. If your writing is naturally lyrical, then don't be afraid to work with words which "renew the world and make the world anew": self-consciousness is the death of this kind of writing, so be whole-hearted.
Make each scene unique. Even if every scene does have a properly-built change between the beginning and the end, it's easy to find that the actual actions are rather similar: there is, after all, a limit to the possibilities. But if you're really clear about what each of your characters wants in going into the scene, and what their point of change is, and where they are at the end in relation to where they started, it should help you to get to the heart of what sex scenes are: the physical embodiment of mental and emotional change. And since that change will be different for each scene, so too should the choreography.
Remember you don't have to write everything, blow by blow, from first kiss to last post-coital cigarette. This is where learning to expand and contract your storytelling, according to what's important, is so useful. Maybe what matters is the end, when he suddenly says "I love you,", in which case all we need of what came before is the crucial stages in his mini-journey to that point, so that we believe he means it. Or so that we know that he doesn't.
Let the reader choreograph the action. The best sex-scenes are the ones which the reader writes in their head. You only need to give them enough of what-goes-where for that writing to happen: anything else may be fun - and let's not deny that sheer fun is one function of literature - but it isn't necessary, and so may weaken rather than strengthen the overall narrative drive. This is where learning to make your Telling Showy is so useful: we get a strong sense of this encounter, in very few words. And avoiding pornography? Porn has only one goal, and it isn't moving the story along. As long as you're telling stories because the story's worth telling it'll be fine.
Avoid the Scylla of too much plumbing: one of the central problems of writing sex scenes is that most of the vocabulary for sex is either medical and Latinate, or silly, or reminiscent of porn. But it's surprising what you can get away with not naming at all, if you're clever with the pronouns. And avoid the Charybdis of crashing metaphors of waves, or other things. As ever with things which are difficult to describe, it really helps if you make your voices as voicey as possible. That will help you make good choices about words and descriptions, whether you go right close in in psychic distance, and exploit stream-of-consciousness, or you step back and let the narrator be the Storyteller. As far as I'm concerned, in The Well of Loneliness, "and that night they were not divided" isn't just prudery, it really is all the story needs, because of what's gone before. The people who prosecuted it for obscenity obviously felt the same.
Remember that what's most sexually charged often isn't sex. As I was dissecting here, in the context of Gaudy Night, it's perfectly possible for the big turning points in a sexual relationship (before, during or after) to be nothing that has any obvious sexual content at all. I learnt as much about writing sex from that book and its sequel, Busman's Honeymoon, as I did from years as a teen reading Cosmopolitan and Judith Krantz's S&F novels.
Buy The Joy of Writing Sex, by Elizabeth Benedict. I haven't got hold of the new edition yet, but if it's as good as the original, it's the most sensible and helpful guide you could ask for. Which essentially means that it's also a sensible and helpful guide to good writing. In the end, sex scenes go wrong when the writer loses sight of their normal, ordinary sense of what makes any scene go right. Hold onto that, as they say, and you'll be fine.