The other thing about it is I've always considered writing combat and writing sex to be the same "kind" of writing, lots of action/reaction structure.
Speaking generally here; your comment there simply sparked something for me.
Sex is far less about mechanics and action/reaction, and far more (as Harlequin said) about emotion. Which doesn't necessarily mean love. It can be affectionate, passionate, adversarial, cold, manipulative, or a power play. It's entirely possible to write an effective, powerful sex scene with little to no mention of body parts and mechanics. Because most of the time, who does what to whom is not really what the scene is about. Above all, it is personal and the experience unique to the participants. Story characters enter sexual encounters with agendas and baggage. The question is, how much of that baggage gets unpacked and either dealt with or left messily displayed, and how much do they carry out again, untouched, to weigh them down another day? And do they acquire new baggage from this encounter?
Interestingly, fights and battles (physical violence, IOW) operate similarly, at least from an emotion standpoint. The encounter is personal to the character, and there are agendas, baggage, and consequences. So it's not just about choreography. Someone's life (or an army's victory) are not all that's at stake or that needs to be explored.