Yeah, at the end, AFTER Sangtae's persistence has made him give in... He says himself that he knew Sangtae wouldn't have stopped and he needed to cum because it "felt sooo good" because he "hadn't been touched in a while"... You're taking everything out of context, the flow of events is important and you're ignoring it.
He didn't "tell him to give it to him", he said "take this" because he decided to get revenge by teasing him, hence his inner monologue right after about how Sangtae's responding to the tease, he watches him. He also didn't "kiss him", Sangtae put his fingers in Jinwoo's mouth and turned his head towards him. He "said it felt good" IN HIS HEAD, WHILE complaining that it's embarrassing and when Sangtae says "you might actually like this?" he says "bullshit! There's no way!", but you'll both twist this so you don't have to feel guilty about shipping them. Lol.

And there (down below) is the wave of people who want to ignore Sangtae's obvious flaws and ship Jinwoo with his rapist... AFTER he said he wants away from Sangtae... "hehehe, after the MULTITUDE of times he's been FORCED to watch/feel Sangtae's fantasies about fucking him in the ass and after he was actually unwillingly sexually assaulted and then raped he feels the need to have something in his ass while masturbating? It must mean it's CoNsEnSuAl!"... I seriously wonder if people have reading comprehension sometimes... Or even just common sense... Let's go over a hypothetical situation real quick: Lets say you have a pet, if every time it shits on the floor you smack it and put it where it's supposed to go, what happens? It stops. Now, let's apply this to Jinwoo. If every time he gets close to Sangtae he sees and feels VERY vivid fantasies about being fucked, what's going to happen when Sangtae sees this, gets horny and then ACTUALLY fingers him? He knows the feeling of actually having something in there now and he's been repeatedly forced to visualize it UP TILL he actually experienced it (the thigh fucking scene as well. "this feels like the actual thing, it's embarrassing"). He's been trained... He has to TALK himself into using the dildo, he doesn't want to. "just once". And he only talks himself into it while he's THAT horny. Can you imagine how weird this must all be to him? He didn't naturally discover a taste for anal play like some straight guys do, he was trained into it. An unwilling straight guy had his body trained to like anal play AGAINST HIS WILL... He isn't interested in Sangtae and now this gay guy that he KNOWS is after his ass has CHANGED HIS BODY. You know how, everyday, masturbation is? You know how WEIRD it must be to suddenly have the WAY you masturbate changed against your will? What happens next chapter must make it worse as a certain someone treats him masturbating anally as a turn off of sorts.