Kimi to Tsugai ni Narenai Riyuu

There was never a time when they didn't like each other - Minato (Omega) and Jin (Alpha) have sworn to each other to become a "pair". However, when Minato was supposed to go into heat, the doctor reveals to him that the chance of him getting pregnant are close to zero... Minato is shocked that is seems like he's unable to bear a child! After all of this he separates from Jin and they drift apart...

This Manga contains 15 years of love between an Alpha who has made a promise to mate, and his childhood Omega friend who's somehow infertile.