Nippon Rettou Gamagaeru

This is a coming-of-age story depicting the days of torment and lust for a young man, Sei, who was instilled with a distorted view of sex at a young age that reminded him of toads' mating. One day, after being told about sex by older boys, the young Sei insisted that his mother would never do such a thing. However, when Sei woke up in the middle of the night, he was deeply shocked to see his mother having sex......!

(Source: eBookJapan Plus, translated)