Zenbu, Hajimete

Haruto is devastated when he discovers his ex-boyfriend is secretly married. Drowning his sorrows at a bar, he meets a man nursing his own heartbreak. They hit it off, and before Haruto knows it, they’re sharing a bed, even though he doesn't even know the man's name. The next morning, Haruto slips away, believing it was just a one-night stand. However, his world is turned upside down when the same man, Hayama Shiori, shows up as his new coworker at his company!

(Source: Core Magazine, translated)

Note: Includes one extra chapter.