Well, I started sketching out the sequel ideas to "Fun With the Colte Family". It's pretty easy to see why those old sleazy Liverpool Press books rarely had sequels! Two reasons why, I think, are basically these:
One: I think most writer's inclinations are that the sequel needs to be bigger. Kind of like superhero movies, where the second film gets two or three villains, instead of just one, and there's more fight scenes. (In the case of smutty stroke stories like these, I guess that means fucking a horse instead of a dog.) Except the superhero movies like that (Batman Forever, Spider-Man 3) typically don't work, at least not when they're equating bigger = better. I think the answer is more different = better. It's not doing the same story over again, just with louder explosions, it's finding new angles and storylines to explore with the same characters. I get the feeling that the one sequel I'm writing will actually be much more low-key than the original story, but I think it might have the potential to be way better, just because it's exploring some ideas I haven't read much in smut stories like these.
Two: You can tell why sequels to pure smut stories like this aren't written much (unless they're just retreading the exact same basic story over and over again). They just get progressively more unbelievable and ridiculous with each new chapter. You can put a family fest spanning a few months in a multi-chapter story, and while it's not totally believable, it's at least vaguely plausible. But over years? And mothers and daughters giving birth to babies, and needing to take care of them? And college? Or jobs? Or the increasing need for improbable coincidence? ("Hey, my new college girlfriend's family just happens to already be totally cool with incest, too! And fucking German shepherds! What are the odds, huh?") The more things bearing a passing resemblance to reality intrude on these sorts of stroke stories, the more they have a hard time holding up. You either have to go full-on break from plausible (they're billionaires! the family lives on a secluded island mansion! they have some sort of life where their secrets will never be discovered, and there wouldn't be any consequences, even if they were!) ... or you need to figure out how to make 'vaguely plausible' work, and that ain't easy.
I'm still giving 'vaguely plausible' a try, though.