How to Write Sex Well
If you're actually interested in writing sex well, I doubt there's a better guide than Resonant's series on How To Write a Sex Scene:
Part 1. Make me yearn. The reader needs to be longing for some resolution in addition to orgasm, so that the described orgasm gets emotional fuel from the drive toward narrative fulfilment along with the physical.
In other words, the sex scene should be part of the *story*, the plot, the narrative. Even in a pure PWP ("Plot? What Plot?" aka "Porn Without Plot" story) there's a narrative, a quality of story-ness the unfolding of the writing along with the shedding of the clothes.
Part 2. Pick one zing and stick to it. That is, a given sex scene can only carry so much weight of meaning and complication, don't try to pack it all in at once. Stick to one major zing and let it carry you all the way for this single scene; other zings, other scenes.
Part 3. Make the sex fit the characters (and not the other way around). This is it, the most crucial point of all. "Don't think about what happens in the scene. Think about what payoff you want from it".
It's all about the characters, because sex is something *people* do. Sex without the human (or sentient alien) dimension is literally mechanical, and (speaking as a biologist) it *cannot* be as arousing as sex with a true person. Not to mention that it doesn't make for interesting writing. Sex as an expression of character is (should be) the Gold Standard for pornographic writing, though I have only seen this explicitly acknowledged for slash. It's quite possible that theoreticians of romance novels, mainstream genre porn, het fanfic, or pro gay porn also refer to this standard, but I just don't know as much about those fields.
Notably, a number of the commenters to Res' post said that even without knowing the show or the characters, this single sentence is an effective sex scene:
"Jesus, Rodney -- say something so I -- know it's you," and Rodney's going to put his mouth right against John's ear and say very quietly, "You know it's me".
[I could give a hyper-detailed analysis of why & how this single sentence works so well, but that might be overkill. Ask, though, and I'll dance for you like a monkey.]
Part 4. Choose your details carefully. You don't have to put in *all* the details, just the ones that: 1. support the yearning you're trying to create, 2. support your zing, 3. tell us something about the character and the relationship, 4. we need to know about it to understand what happens next, and 5. really turn you on.
Just as mathematicians historically felt uneasy about Euclid's Fifth Postulate, I'm not so sure about this Fifth Principle of Writing Sexy Details. What I too often see is a writer being so distracted by what turns *her* on that she forgets the other factors, especially the question of whether this is a turn-on for *this* character in *this* circumstance. I suspect that the "what turns you on" decision needs to be made earlier, in the yearn/zing part of story development, so that when you get down to details the characters tell *you* what turns them on, instead of you telling *them*.
And if the characters never speak to you? You probably shouldn't be writing about them.