If you're talking about this "Reworked sprites for the Fly sex animation 2." in my mod, it's basically the edit I showed
here (the first one, the others aren't in the mod).
Let's not be too confused about this.
What is complicated (mainly very time consuming) is to add brand new enemies because you need to write a lot of code for them and integrate them within the already existing ecosystem of the game.
Editing the already existing sprites is fairly easy, but depending on what you want to do, it may or may not be a long process. For example, what was asked earlier about making the player character fully naked is perfectly doable, but it's huge task to go back on every single sprites and make the character fully naked (you roughly have to redraw 50% of the character from scratch on each sprites).
That said, adding new sex animations (so new sprites) for the current enemies already in the base game is something that should be doable without being as much of hassle as adding a brand new enemy. Still a kinda long process though.