a revelation
I said this in a comment-reply in my previous post, but I'm going to promote it to full status here:
Sex scenes are no different from fight scenes in terms of writing them. You still need to talk about what the scene means to the respective characters, what they're feeling (fear/anger/elation/determination/etc in a fight scene, [similar long list of options] in a sex scene).
You still need to keep track of where people's bodies are so they don't do impossible things.
Typically you don't get into the nitty gritty details of either one, unless writing porn (violence porn/grimdark on the one hand; erotica on the other).
Wow, I just realized that grimdark is the fight-scene equivalent of erotica.