I'm not really sure about this, its been long since I read that part of the story but, if I remember it right it was because maya and nemu's relationship was going smoothly and he was jealous and frustrated that his and akira's wasnt? ah, I feel like I need to reread tthe whole manga. ive forgotten a lot of things already. This may be wrong so guess you'll just have to wait for another response (￣∇￣")
Oh, thank you. You most likely are totally on the mark, I wouldn't put that past Naoya.
Just to add a little bit here to the previous answer: after Akira and Okino left to get drinks, Akira had left his phone, and he got a message from his old sensei. I think the message was like, "thank you for meeting me yesterday", or something like that, I don't remember exactly; Naoya got even more jealous because he was under the impression that Akira had moved/was moving on from his feelings for sensei, and Akira and he(Naoya) were moving forward. You get Naoya's perspective, specially this part, in the Akira Story because here in Hidoku Shinaide, he just kisses Nemu. I think he wanted to make Akira jealous but not really hurt him or Nemu or even Maya. Naoya is just a man who gets jealous super easily; he would even get jealous of Akira's closeness to Maya.
Question, I never understood why Naoya kissed Nemu-chan, can someone help me understand what happened?