Actually, in the novel, Wooyeon's thing is flaccid but still when he shoved it in Inseop's mouth, it still hurt because he's technically still big down there(but not same as shown here) plus he's getting aroused when he first did that to Inseop.
I actually felt bad that the artist had to redo this part just because some fans complained about the size despite it's how it was described in the novel making it looked not normal at all. Basically, his first man was Inseop and unexpectedly he got aroused is what should be portrayed. Point is not anyone will do, he became like that because it's Inseop but some had to make unnecessary complaints. Making Wooyeon look like a man who only cares what's inside his pants.
This guy is already a red flag since the beginning so there's nothing surprising here. Pls. Remember he has mental problems with no one to manage him.
Anyway, in this situation, Inseop still suffered. Too bad for him that he crossed paths with Wooyeon and Jenie. But it's like he was really meant to meet these two individuals.