Boku wa Shitteiru.

High school student Sunao was commuting to school when he noticed a salaryman on the bus. On his ring finger, were teeth marks hidden by a ring. Who left those marks? Is it a sign of possession? Or maybe... Before he knew it, Sunao's head was filled with fantasies about him. One day, he runs into the salaryman at his part time job at an izakaya. He drunkenly confronts Sunao and kisses him...!? These straightforward and painful emotions are too young to be called love. A love between a high school student and a salaryman!

(Source: Hakusensha, translated)