Hakudaku Musume ~Ki ni Naru Ano Ko ni Marking~

The protagonist is someone who often lusts after girls, but unfortunately, he isn't very popular. One day after school, he finds his classmate, Mikoto, asleep in class. Overcome by temptation, he decides to dirty her with his semen.

That night, he is too excited to even sleep. Moreover, on the next day, she doesn't seem to notice his act. After opening his eyes to this new fetish, he vows to do the same to the girls around him.

[Translated from Official Site]