Kobito Ijou no Koto wo, Kanojo ja Nai Kimi to.

Yamase Fuyu, tired of his black job, runs into Ito, his ex-girlfriend from college, one day and invites her out for a drink. They talk about their old days and their new complaints, and just as he was thinking that this is a nice kind of friendship, they get carried away by the energy of alcohol and end up having a one-night stand. Fuyu regrets what he did, but Ito tells him, "Don't apologize. Don't say, 'Let's get it right.'" Ito, too, was tired of being forced by society to always act properly and adult-like. Thus, the two form a mysterious relationship where they only do fun things, without worrying about work, love, or other troublesome annoyances.

(Source: Shogakukan, translated)