It is highly moddable, in fact, I've done a bit of modding on the game myself (I've now dropped the mod 'cause RL SUCKS) and I can say that, YES, you could add extra sex scenes, but the main issue with that is you'd basically have to reverse engineer the (Said by dev) spaghetti code to figure out how he implemented said scenes into his script, then trying to add new ones on your own. Most likely you would need to create custom NPCs that are linked to the story, not auto generated, and then when you finish debugging THE CRAP out of said extra NPC you then could add new scenes to that one, either that or add new scenes for preexisting NPCs which, again, would require you to go down a rabbit hole of coding that most people can't be bothered to do.
So it's less "Why hasn't no one done this?" and more of "Who is WILLING to do all that?" The answer is few and far apart. Also, people (Like myself) are waiting on the dev's new game, which should be an improvement, so they don't bother with modding.