After treating Taehyung as a slave for short.
On knew that Taehyung was also hurt by him, and knew that he were Jungkook's husband.
But Taehyung already divorced with Jungkook so On decided to be with Taehyung.
Jungkook didn't know when it happened. There wasn't a single definitive moment that made him realize that he wanted more. He just knew that he did. He wanted Jimin in a way that he shouldn't want him. A way that a dongsaeng shouldn't want his hyung.