His First Love, Poison Or.

It's been ten years since Miyako fled from feelings about his childhood friend. Only the invitation to their wedding brings the two back in contact with each other. While Miyako is still trying to forget his unfulfilled love finally, another man suddenly appears on the scene. Yuya, the younger and most attractive brother of his childhood crush, admits Miyako his years of affection and thus ignites a storm of old and new desires...

(Source: KAZÉ, translated)