Yoru ga Hodokete Hana ga Saku

At the beginning of his last year in college, Shizuka meets Izumi, his new professor's assistant. Shizuka is drawn to Izumi's black hair, pale skin... and seemingly lonely expression. However, he realizes one day that it's the professor who is the target of Izumi's gaze. "...The person I like has someone they like. And that someone has a wife." It's so painfully pure and maddeningly sad. It's a romance between guys that cultivates love with great care.

(Source: Renta!)