Right up front, let me say, I've long preferred writing action scenes to sex scenes. Partly, this was due to me having absorbed some remnant of Puritan social conditioning - you know the one: Nice girls don't know anything about sex, much less talk about it? I have no clue where I picked up this nonsense. But the fact remains that to this day, writing gun fights, hand to hand combat, sword fights, explosions and assorted spaceship battle scenes is easier than writing a sex scene.
But it doesn't make sense for one scene to be easier to write than the other because a sex scene IS an action scene.
No. Not that kind of action - well - okay, yes it is *that* kind of action, but you know what I mean. Look. A fight scene arises out of the external conflict of a story, it may be fueled by a character's internal conflict as well, but usually a fight comes when our heroes have been backed into a corner by the bad guy. The action of the scene will either reveal something about the hero(es), the villian or the core external conflict of the story. The fight will probably propel the hero(es) closer to some kind of doomsday confrontation - not just with the bad guy, but also with themselves - this proves they've completed their character arc, learned their lessons and now have the wherewithal to triumph.
A sex scene arises from the internal conflict of a story. The external conflict may provide a sense of urgency, or heighten whatever is driving these two people together, but the sex scene serves the same purpose a fight scene does - driving story arc, character arc and plot. It's another case of proving that the characters have learned enough about themselves that they can risk trusting someone else. It's also likely that these people will emerge from the encounter either shattered or strengthened - which is usually how a fight ends, too.
A good fight scene can utterly disarm your character, leave him or her stripped, defenseless. So can a sex scene. A fight scene can empower a character, girding him or her for the final battle. So can a sex scene. From a staging standpoint, there's no difference at all between how a fight scene and sex scene must be directed - only the anatomical language changes subtly - unless the sex scene is a fight scene. Combat is a contest for physical supremacy. A sex scene (not talking rape here - which is 100% violence, not sex scene) can be as well, and while a story wouldn't likely be a romance at that point, there can be an element of competition to a sex scene - who makes who climax first.
Regardless, knowing what limb is doing what, where, when, and knowing that all parties have an appropriate number of limbs is the same whether you're writing combat or sex. The differences in emotional content can be huge or subtle - depends entirely on your characters and on the story. But ultimately, there's no much difference between the scene types. They both require the same set of skills.