You've just told the naked truth: 99,999999999% of Adult Visual Novels are a complete failure.
They're supposed to be erotic fiction and they fail both as erotic and as fiction.
I wouldn't be this extreme even if, as a matter of fact, a good 90+% of games, especially recent now games, are indeed lazy garbage with 0 effort taken and visibly so.
But if we stick to the games that are well done, or at least decently done, even when the story is actually good, and there are a a good deal of games with a rich well conceived and well executed story, the plot and the sex elements feel disjointed.
Like "Hey! there is this mysterious serie of events, possibly dangerous implication and very shady actors! We need to investigate! But just give me 15 minutes, right now i need to fuck my cousin's brain out, then we can go on with the investigation!"