Bad sex scenes are fun to make fun of but I think one of the easy reasons they are frequently awful is that there's no narrative energy to them. I picked up on this by studying combat scenes and I think the problems that show up are really similar; there's this thing writers do where they treat these kinds of sequences differently than the rest of their work, the scenes feel like they exist outside the regular narrative and writers just pause the story while they happen. People are always complaining about fighting and sex in stories being "boring and a waste of time" and that's not an inherent function of these mechanics, that means writers aren't approaching them with the same thoughtfulness as the narrative, and the audience can feel it.
The sex is an interaction, and interactions should illuminate things! It's still beholden to the plot/characterization/worldbuilding rule, we should always be communicating some kind of new information or reinforcing what is already understood. You are having 2+ personalities strip down and communicate with their bodies what are they SAYING. It doesn't have to be anything profound, even just communicating to the audience "i think this person doing this thing is hot" counts; wanting the reader to get off to something is a perfectly respectable goal that can keep us on track if you're aware of it. But the writer so frequently has NO IDEA what they're actually trying to say, what the goal of the scene is beyond fucking or fighting, so we end up with bland interfacing between characters who could be replaced by any other characters doing the same thing anywhere else. It isn't hot or interesting to be directionless, pick an intention and go.
