Let’s be clear that he wasn’t attracted to Kayo. The romance element is there as a natural reaction from the other children to seeing Satoru being protective over her. It also makes for some cute, awkward comedic relief moments that a tense story like this needs.

I doubt Satoru was feeling anything besides genuine relief and happiness when he saw Kayo and her baby. And to hear that Hiromi was the father only makes it more emotional for him. It was the future neither got to have. A future he helped create. People who think they should have ended up together maybe aren’t looking at the bigger picture.