Yuganda Ai Shika Shimesenakatta Ore o Yurushite.

--the protagonist, Nishina Sougo, is in a friends-with-benefits relationship with his childhood friend, Kaihou Tsumugi.
Sougo had just recently gotten a girlfriend, and wants to cut off his and Tsumugi's distorted relationship, but due to some past circumstances, he can't do it.

After dragging this relationship along day by day, Sougo discovers something unbelievable.