Koibito Ijou no Koto wo, Kanojo ja Nai Kimi to. @comic

A fun, carefree, and casual relationship.

He spent the night with an ex-girlfriend. Exhausted from his demanding job, Fuyu Yamase reunites one day with Ito, his girlfriend from university, and they go out for drinks. As they bond over complaints and old stories, Fuyu thinks how nice it is to have this kind of friendship. But in a moment of drunkenness, they end up sleeping together. Although Fuyu regrets it, Ito sternly tells him, "Don't apologize." She continues, "Don't talk about doing things properly. Let's just stay together because it feels good." Ito, too, was tired of the societal pressures to be correct and the constraints of adulthood.

Thus, they form a peculiar relationship: excluding bothersome things like work and love, and just doing fun things. A short story series offering comfort and a touch of eroticism to those weary from society.

(Source: Shogakukan, translated)