I think the story development issues need to be taken with a grain of salt. What exists for RenPy, Unity, or RpgMaker are greatly more nuanced than the porn you will find on Spankbang or Pornhub.
If you look around there are a few decent production groups but all with the same tropes. Even the best production group, TeamSkeet in my opinion, tells stories in two or three parts. There are step-sister or step-father videos readily available with anyone insinuating a blood-relation to characters usually coming from small production groups.
There is always a corruption phase: where sister or daughter is either naive sexually & doesn't know what a penis is, or the opposite and is a whore guilty of something. The male is aroused by their relative and that becomes obvious to the actress who offers a brush with forbidden fruit in return for something.
An escalation phase: where one of the characters approaches the other and asks for more sex acts in exchange for something else. This usually results in the guy realizing that this doesn't need to end, and the actress coyly admitting to enjoying everything.
The final phase is best described as a wish fulfillment "my private slut": which is just like the description for this stage. A final scene where the two are placed in a room and go at each other. Both confess their primal urges and see nothing taboo with continuing this part of the relationship purely for sexual gratification.
We don't get too far from these story tropes in games, but we try. As entertainment they should be considered for examples. By observing what other media with similar premises can accomplish, & where they fall short, we can adapt and improve this form.