Bro are you fully aware that what you are searching for is an eroge game and there is a shitton of them?
Basically games which are talked about here and played are mostly nukige games, games where the story and setting is used to get a background for the porn, what you search is eroge, games which focus on story while adding adult content as flavor for mature audiences. Someone already mentioned several eroge VN like Fate series.
I do recommend you check :
-Saya no Uta, thats one of the classics, its one of the best horror eroge out there,
-Katawa Shoujo, a sim date eroge, where you date disabled girls, its quite good and has some good story
-Kamidori Alchemy Meister think KOEI TECMO's Atelier series but with erotic content
-Eiyuu Senki, tactics RPG in alternative world where all know heroes and generals are female, erotic content happens here and there but most of the time its not as part of the main story but as part of progressing the relationship with characters.
-Rance Series
-Grisaia series
-sono hanabira series, if you want yuri (lesbian)
-Seinarukana, this one is one of the masterpices, it has all, good story on the level of classic jRPG like Final Fantasy, with romance, drama tons of epic action.
-Bunny Black 1 and 2
-Utawarerumono
-Tears to Tiara
-11 eyes
-Dies Irae, one of the best know eroge, its a masterpiece which was re-released so many times that even bethesdas Skyrim re-releases pale in comparison
Basically around 70% of the VN on steam are eroge, but as most of the bigger eroge titles, they get a 18+ version and a all ages or 16+ version depending on the content. For a quick example, Dies Irae without erotic content is 15+ cause it still has all the gore in it.
