Stringendo: Angel-tachi no Private Lesson

Both are hentai anime that portray stories of various couples. The pairing stays vanilla for the most part (Stringendo episodes 2, 3, 6 contain non-vanilla). Both series showcase really aggressive yet likable girls. The animation is well done as the camera angles frequently change, allowing us to have as much imagination as we want. 

some of the best (sexiest) character designs and hentai animation. 

Focus on romance between pairings (through sex obviously). Mainly consist of vanilla main guy x main girl pairing. In some episodes there are couples that are NTR'ed. In the end the story always go for the happy ending. 

Kateikyoushi no Onee-san 2 The Animation: H no Hensachi Agechaimasu has adapted to using similiar animations as Stringendo(plus all the ones related to Stringendo).  

Both animes were based on the original stories of Yuki Seto. Stretta contains mixed characters added from other series with some adaptations to their roles (or the setting), but their main focus of course is still the same.