I hate to do this again but I have to state my opinion. I agree with what you're saying that Euntak is basically naive for expecting a f*ckboy to change for him but I think he has the right to express his emotions and getting angry despite yes he was really naive.
but. I think Euntak does not have to spell every single word to Dojae so that he understands that “don’t sleep with other people” meaning do not sleep with other people. Dojae has all the right in the world to refuse Euntak’s whole proposal if he doesn’t agree but he didn’t. That’s why Euntak EXPECTS Dojae to understands that he wants “monogamy”, just for the 3 dates, just for that.