
Id honestly say it has a lot to do with homophobia and toxic masculinity. Dan is obviously the more “feminine” of the two so he’s portrayed as the full gay one. Jk on the other hand is supposed to be the “ perfect alpha male” so he’s only bisexual most likely. Tbh this isn’t even the first time the author had pulled this kind of thing, it seems rooted in the idea that a man can’t be both gay AND masculine.

He was actually bisexual but more attracted
to men, we could see him watching p*rn in few panels to help him during broadcast when he couldn't get it up and it was straight p*rn, DG as bottom was gay, MD was bisexual without preference and Chanwoo was pansexual.
This is stated at final author's note of BJ alex.
I can't imagine Joo fucking a girl, mingwa can't make characters fully gay for tops just for bottomsc slightly homophobic