You really REALLY like justifying ra** bro, Dan wanted to stop before he even started the s**, but he was scared to ask because he knew Jaekyung might end up hitting him even more than he hit the other bottom, however when Jaekyung wanted to get in Dan pushed him and said "I'm really sorry but I don't think I can do it"
which obviously meant he didn't think he could go through with it anymore, and that's also what Jaekyung understood, that's why he threatened Dan to stay, and Dan who was already scared before got MUCH more scared after that, he literally started shaking with fear, and just then he asked "then how do i do this?" since he had obviously given up on leaving after that threat.
Besides, after Jaekyung got into Dan couldn't take it again and asked him to stop again, saying "Stop! Don't!" and once again Jaekyung responded with a threat 💀