I think the top, cause he said mc has put his mark on jin but not him...it also kinda confused me, so I'm not 100% sure
First of they're all toxic lol. Ido does like them both for different reasons. He likes mc because mc loves him regardless of all his flaws. I think Ido has abandonment issues. Ido likes Jin because Jin is a passionate and talented person. That's what drew Ido to like him. Kinda like he's living vicariously thru Jin. This chapter he sees them together and feels like he being left out and is insecure. Like they might start to love each other and leave Ido.
I agree it's the top he's jealous of. It seems to me that he is use to having Gyeonwoo love,kindness, & submission to himself because he kept thinking about when did they become close. It seems like both partners have something that balance something within him which is why he is pushing hard for this thruple. However, I don't think he realize how hard it'll be. One side of him does want them to get along and be in love too but another side of him is worried that he'll eventually be pushed out and they aren't going to want him anymore.
I think he’s jealous of just them having a relationship, but more so because he feels like an outsider for first time or something.
Ouuu that could be it
wait what’s he mad abt..??? is he jealous of the bottom or the top