Lick.

Synopsis

Toame is a high school boy who just started going out with his first girlfriend ever. He should be happy, but his expression is still gloomy for some reason. In fact, he already has a "secret" relationship with Nameki, the most beautiful girl in his class, which he cannot tell anyone. Toame wants to get out of this toxic relationship with Nameki, but he finds himself sinking deeper and deeper into a quagmire... (Source: Bessatsu Shonen, translated)