I think of it as brotherly love because he lost his brother so he had no one to look up to. He looks up to Otonashi now because he is a kind person and thinks of him as a sort of older brother. He is most likely clingy because he lost people he cared about before those events. I know from experience. I lose my friends because they either move away or go to a different school. Anyway, this is just a theory of mine that I would like to share to support that Naoi isn't exactly gay. Hope that this helped.