Akatsukino Omega Kimamana Neko Ha Tsugawanai summary:

Society has discovered that in addition to alphas and betas, some people are born as "omegas." However, omegas face severe discrimination. Abandoned by his parents at birth due to his omega status, Sei grew up in an orphanage, but he escaped as soon as he reached maturity and started living free, like a stray cat, off any man he could find.One day, Sei bumps into a childhood friend he hasn't seen in forever, Yuki Omiyata, a.k.a. Omi. Somehow, this leads to him moving in with Omi! For the first time ever, Sei's living a wholesome life in a stable home. Not only that, Sei realizes Omi's been harboring feelings for him all this time, and finds himself attracted to Omi, too. But, one day, Sei finds out he's got a baby of his own growing inside him...!This spinoff weaves a tale of young love, taking place more than ten years earlier than "The Siesta Mates."