Oneesan wa Suki desu ka?

Ryoutarou Sakurai is a 29-year-old office worker at an architecture firm. One day, he is asked to visit an architect's office, and who should await him there but a beautiful, mesmerizing young woman... No, wait, young man?! When the up-and-coming young architect Hibiki Fujimoto takes a liking to him, Ryoutarou finds himself reeling at the mercy of Hibiki's frivolous and freewheeling antics. Not to mention, when Hibiki makes advances toward him with that pretty face, he just... can't...!!

(Source: Renta!, edited)