I didn't get the end

Okay so this man that Kusaka called his "father" was in love with his biological dad, okay, I got that but I thought Kusaka's biological parents died so that's the "father" took care of him, but then the biological parents appear in chapter 5.5?? so he was just a close friend of his family???? so why Kusaka's said that Mikuriya is all that he has??? I thought he said that because all of his family died, and why would Kusaka told Mikuriya he thought that man was his bio father but then he said he immediately realized that he wasn't his bio dad but he was in love with his father????? lol I got confused with that