The thing you posted is shit and applies real world logic to fantasy. Drop it, it's nonsense; do whatever wacky bullshit your heart desires.
Instructional images/serials like that are almost always reductive and needlessly constraining. Read the lit or do a master study instead.
First off, two things:
There is no easy answer or method for what you are asking. There is no "do this to make something beautiful". You will have to figure that shit out as you go. This extends to my recommendations, or any reading on art. They contain techniques and methods, that are useful to achieve a result, but are not law.
Two, there is no "advanced" mechanical art theory. The baseline is the theory. However, that baseline is very, very broad and encompasses everything from anatomy, to colour theory, to material studies, and so on. What you have been introduced to is the extend of the playing field. Now you have to master it.
The only hard rule I can give you for smut is anatomy. Tons of it, even (especially) if you end up simplifying it. That is, you need a VISUAL memory of contours, which is given by bones, muscles, etc. The shit you have been thought.
Other than that a general knowledge of human anatomy (where organs are at, like you posted) is useful but not required. Cheating is always better if it is more efficient or more appealing.
Check out Michael Hamptons Figure Drawing - Design and Invention, and I can recommend Stephen Rogers Pecks Atlas of Human Anatomy for the Artist as a general reference. It's possible the former doesn't work for you, so note down Gottfried Bammes' The Artist's Guide to Human Anatomy and Michael D. Mattesi Force - Dynamic Life Drawing for Animators. I'd caution against Burne Hogarths writing on anatomy, though they're an interesting read if not very useful practically.
Also teach yourself animation if you haven't yet, as the understanding of motion it will give is invaluable. Fucking necessarily involves motion, and knowing how to make that look appealing is tantamount.
Take a look at Preston Blairs Cartoon Animation and Richard Williams The Animator's Survival Kit. Don Bluths The Art of Storyboard and Marcos Mateau-Mestres Framed Ink are more tangential, but still recommended, particularly if you end up making panels (which you will).
None of these will teach you how to make something beautiful. That is all you. Therefore draw, paint, and above all steal from works that appeal to you. Only you can add beauty to a work.