Kore wa Uso no Kekkon Dakara

In a world where same-sex marriage is legal, 28-year-old bachelor Mashiro Kotobuki has built a reputation as a top-notch matchmaker at Mari's, a leading marriage agency. Despite his knack for helping others find love, Mashiro actually has no romantic experience of his own! The CEO, his mother, is constantly on his case about settling down. One day, Kaede Narita, his average Joe coworker, walks in during a nagging session and—out of desperation—Mashiro blurts out that they're dating! Mashiro is then left on the fence when he is offered the chance to head a new same-sex marriage agency if he can actually tie the knot. From there, the two pretend to be in a chaotic "fake marriage," but it appears that Kaede is hiding something...

(Source: Gentosha, translated and edited)

Note: Includes one extra chapter.