Anakoi ~Koukousei Ren'ai Hakusho~

Kanata, the protagonist, has been pushing away others so that he does not become too close with others. However, by a twist of fate, he is asked to help Madoka, a naive sheltered girl, to do detailed research on the anal sex. At first, it is just some touching with fingers. But gradually the girl finds pleasure in it and the practice gets harder and harder.

[From DLSite English]