I re-read it to jog my memory in case I was missing your point, during the first scene, J (somehow) does not realize Dan is a virgin and even believes Dan is well-experienced and so he doesn’t feel the need to ease into it, it’s bazar that he didn’t realize as all the signs were there but it seems that was the author’s intent, allegedly. As for the threats, they were made on the baseless assumption that Dan was messing with him, trying to provoke him, from the beginning, Jaekhyung is introduced as being highly defensive and rash, that being said, throughout the scene he verbalizes his intentions beforehand and says Dan should leave if not ok with the terms. Also there are frames where Dan indirectly expresses pleasure as “tingling” inside and questions how it is possible for him to feel pleasure through penetration. Of course there is nuance to it but my point is Jinx is dubious consent, which understandably is not for everyone and that’s valid.
I just have to keep reminding myself that this isn't a actual BL but a smut, smh. If he goes back to that man, I'm gonna kms. Also, the people that don't want Jaekyung to suffer any more, simple words, fuck you.