Both the uke and seme cheat on their partners with each other. But both have a long history with each other and were each other's first loves and could not forget each other. It's a story of fate and Destiny and how they both tried to move on but couldn't. Yes, they could have come clean to their partners earlier but guilt especially on the uke's side silenced him. Eventually, the uke just couldn't forget how strong his feelings still were for the seme and although he tried to move on from him, he could not and he broke off with his partner telling him everything.
Wonder Border