I believe that the quality of storytelling has declined, not primarily because of the writing itself, but rather due to the influx of games that recycle assets or are blatant clones, created solely for quick revenue generation. As a consequence, there's less emphasis placed on crafting good...
I don`t think there are turn based porn games specifically, either existing games with mods or just management/rpg, something like Free Cities: Origins.
I think the first actual game was "Dating My Daughter" that got me hooked on porn VN`s. Before that played a bunch of flash games but can`t remember their names.