You said it right he WAS a black belt...like years ago....if you ever went to the gym and got sick or something you know muscles fade really fast, and with them the power, this guy probably doesn't enter a fight for years and even if he was still a black belt one thing is a ring with rules a begging a end, predetermined moves....and he is obviously emotionally shaken....I heard of black belts totally freeze in real life situation, people who blame themselves for literally "let themselves" be raped, I think it is pretty clear MC just don't know how to react, and never was a violent person (I myself am tall and strong but my response to any panicked situation is also freeze)
bruh...do u see how big seo-in is? And some chapters ago he forced mc so easily, bc he wanted to rape him (until mc punched him)
Bro was a black belt, he could literally body slam the ml anytime he wants bit this mf keeps acting like a damsel in distress. Fuck that