Kore wa Uso no Kekkon Dakara

This is the story of a world where same-sex couples can marry. Mashiro Kotobuki, age 28 and single, works as concierge at "Maries," a marriage matchmaking service run by his mother, where he is well regarded by customers for being considerate, attentive, and honest. But in reality, he is a complete novice in matters of love... The president (his mother) has often nagged him about getting married, but this time his heart wavers when she informs him that, if he gets married, she'll put him in charge of a new enterprise, a same-sex marriage advice agency. Mashiro drags in Kaede Narita, his mild-mannered glasses-wearing colleague of the same age, who happens to be there, and declares, "In fact, we're dating." From then on, they act out a tumultuous fake marriage, but it seems Kaede also has a secret...? A marriage love comedy about a mismatched couple that begins with a lie!!

(Source: Gentosha Comics, translated)