A few thoughts on the subject
Let me start of by explaining my "credentials" regarding this:
I'm somebody that has studied biology, psychology, anatomy, and most medical disciplines that involve both the human mind and human body. And no, I don't have a degree in any of 'em, just a half-century or so of amateur study. I certainly don't claim to be any kind of expert in any of them.
The reason for all this studying is that I got involved in martial arts quite early in life, and I liked to draw and paint a little. Usually people/figures.
Both of those things require a pretty extensive knowledge of how we humans are put together.
The drawing and painting require it so you can construct a figure that looks like it should. The martial arts require it because it's a helluva lot easier to take something apart when you know how it's put together.
The psychological stuff comes in handy for knowing how a person is likely going to react to certain stimulus, either physical or mental.
You can make a person behave any way you want if you understand what you need to do in order to get the desired results.
Play on their mind, you'll likely screw up what they and do with their body. Play on their body, their mind likely won't work properly.
Got it?
Okay, now here's a weird fact for you... You cannot induce most psychological reactions through strictly physical means. You can't make a person afraid through pain alone. It's the emotional reaction to the thoughts of that pain that cause the brain to overload and shut down the body... at least until physical shock sets in. Proof of this is the number of people who had horrendous wounds but kept functioning right up to the point that all physical resources were exhausted.
Sex, on the other hand, can cause changes in the mind simply by physical stimulation that at least at first, has no emotional impact at all. Chemicals cause the whole system to respond. And the same is true of mental stimulation... it sets off the physical chain.
So, unlike writing a fight scene, where you can separate out the mental and physical fairly easily, you'll have a near-impossible time doing that with a sex scene and make it believable. You just can't separate the mind and body with sex the same way you can most other physical activities. Well, not with "normal" people anyway... but that gets into a whole 'nother subject.
In the end, when you start writing about a sexual encounter, you're going to have a lot more there that you can and need to describe or dissect than you will with say, a sword fight or a bar room brawl. Even though some times one looks very much like the other, at least on the surface.
I hope this helps and makes sense, and I apologize if it only muddied the water and made things worse. I understand this stuff much better than I can explain or describe it... which is why I've always been a good student but a poor teacher.
If you have any questions, ask, and I'll try to do a better job than I have this first time 'round.