I think they do but only in the extras. Juho was not exactly in love with jiin he just took him for granted and thought he was a kindred spirit. In general this guy treats everyone like shit and is kinda paranoidly protective of his feelings but he had a freak childhood so it makes sense
I'm taking it back they don't, there's no real jealousy arc regarding jiin and his bf, mc meets the bf in France in the extras. (Jiin introduces him.) And if I remember it right Juho was elsewhere doing fuckall (well not actually but it is a spoiler)
Oh,tysm for the spoiler. Cute they show up as a pair later on.
Spoilers plz, does the ex and his partner come back into the story later on or are shown interacting with the main pairing?