From what I got without re-reading, so maybe forgetting a few details: Seme had the hots for Nemo-kun since he was basicly the only classmate that tried to talk to him, even if it was to keep a false goody goody image. But since Nemo was really popular and wasn't sincere towards him, Seme couldn't approach him and deepen their relationship, that is until he hears the convo about his small penis. He then has the idea of shaving his pubic hair and lie that he had some sort of disease. This made very easy for him to make his move on Nemo-kun under the premise of "training his penis to get bigger because he had his share of problems". The magazine thing was used to lury bully-kun into picking on him, so that Nemo would go to his aid.
glad to know that i wasn't mistaken in my conclusion- it made me like the seme more so i was hoping i was right about the fact he planned everything and he really as pathetic as he made himself seem.
thank you for the summary. now i get it
i still don't get the ending of the manga tho. Why is the seme manipulative? What does the flashbacks tells us? Can somebody explain it to me? (sorry guys, sometimes, i'm slow in realizing things T-T)