Kinyoubi no Toshishita Kareshi

Heartbroken Hirotogawa drunkenly accepts a marriage proposal from Kudou, a junior colleague. Kudou takes Hirotogawa home and spends a wild night together, but Hirotogawa, with no memory of the events, completely forgets about the proposal. The next day, he’s stunned to discover that his application for a same-sex partnership with Kudo has been accepted by the company..?

(Source: Core magazine, translated)

Note: Includes one extra chapter.