He said "take THAT", he's NOT saying fuck my ass... You fu- read back, for the love of god read back. I feel like i'm screaming into the void. If you understand the flow of events, he is clearly getting back at him in the only way he can think of with the low amount of control he has over his body... He says "take that" like if you pinched someone and said that.
When he was fingered he was THROWN DOWN into the stall and he yells "what are you doing". Jinwoo told him at the end of the 1st scene that "the fantasies stop when you touch me", meaning Sangtae KNOWS that all he had to do was something simple like put a hand on his shoulder, but instead he watches Jinwoo squirm, get horny and then literally throws Jinwoo into a bathroom stall and starts sex with him instead.
I say "tease" loosely, it wasn't light hearted, he was bitter and angry at Sangtae and looking for a way to get back at him for revenge. He needed to cum anyways and he knew Sangtae wouldn't stop so he gave in, but before he did he got some revenge and "teased" him. Idk a better word. Again, the kiss was forced on him by Sangtae... You SEE Sangtae put his fangers in Jinwoo's mouth and then they're kissing, he turned his head and frenched him, hence why you can see Jinwoo's tongue.
Right before the "tease" Sangtae says "could it be you actually like this" and Jinwoo says "bullshit! There's no way" after having JUST said "why'd he focus on the tip, the amount of pre-cum is embarrassing". So, he talks about how he feels good "because he hasn't been touched in a while and that it's embarrassing, THEN Sangtae says he actually likes it and he exclaims "bullshit! theres no way...", and THEN he "teases" him because he's pissed off.
There is no point is arguing with someone like that, don’t argue with a person that says there is teasing in rape.

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.