Aitai Jou to Usagi no Yamai

At the Engetsuza Shrine, there's a magical tale that's been passed down through each generation about what happens when a male descendant falls in love. The priest's son, Kounosuke, is a very ordinary high school student. Every day he takes care of his childhood friend, Shuu, a relax and laid-back guy. When suddenly one morning, Kounosuke realizes he's in love with Shuu...and a strange change to his body starts to occur.

(Source: GOT, translated)