Ch. 14 was when it was mentioned. It was around the time where Motochika was also feeling uneasy about the future of their relationship since Natsuki decided to stay with his mom, so hearing the story about Natsuki letting a random classmate pierce his ear set him on edge and made him jealous (it's essentially a daily reminder of another guy leaving his "mark" lol). So the fact that Natsuki was able to coincidentally meet up with the same guy again years later and even attend the same school as him was triggering for Chika lol But no, Natsuki and that guy never dated or anything.
ohhhh now I get it, thank you so much!
Which chapter was the piercing topic? I don't remember well what Natsuki said about it and now I don't understand why Chika is jealous hahsjsjhs did Natsuki and the dude who pierced him had something?