My Boss is My Stallion!

Kotoka is the only daughter of the Hoshou family, a prestigious family that has only produced female heirs for generations. From a young age she was told that she would take a "mukoyoshi," a centuries-old Japanese tradition of a man being adopted into his wife's family as the heir to continue the family line. Before she knew it, she was already in her mid-twenties, and her mother arranges for Kotoka to meet her potential suitor. But it turns out to be that man from work?! The one her coworkers say is "the last man on earth I'd want to marry"?! Although his attitude and choice of words when they first meet is concerning, she decides to go along with it for a little while, confident that she can turn him down anytime. To her surprise, they get along well and thus their life trying to live together before marriage begins!

(Source: MangaPlaza)