Tsukiattenai no ni Fuufu Yattemasu.

Kiichi Amakasu is a gay man and it's been three years since he started living with his straight best friend, Ryuunosuke Shibata, while raising the latter's niece, Mako together. Even though he sometimes has trouble balancing child-rearing and work, Kiichi still managed to build a family-like relationship. But there is one thing that he's been hiding and that is his years-long unrequited love for Ryuu. To make sure that Mako has a happy childhood, Kiichi had planned to keep his feelings hidden forever but one drunken night, he ended up sleeping with Ryuu?! This a heartrending story about a painful unrequited love and family bonds.

(Source: Houbunsha, translated)

Note: Includes an extra.