I get where you’re coming from, especially about Muheon not wanting to dive headfirst into a relationship with a problematic age gap and power dynamic. But I think Seugjoo is being more mature here, especially given his limited experience, because he likes him but seems to be explicitly against getting into a relationship with Muheon—and his cold/passive behavior has remained mostly consistent with that mindset (obv except when he’s drunk), I mean he’s been trying to cut ties with Muheon multiple times already. Muheon on the other hand has been EXTREMELY wishy washy, and him forcefully kissing Seugjoo right when Seugjoo is trying to put an end to their situationship is emblematic of why everyone’s frustrated with him. It’s also very manipulative to use Seugjoo’s drunk advances not only as an excuse for intimacy but as a way to place the guilt on Seugjoo for initiating that intimacy. Like you said, the younger person should control the momentum of the relationship, and Seugjoo is trying very hard to do that; if anything, Seugjoo is trying to END that momentum, and Muheon is being immature by not letting it happen all while being very secretive about his intentions. Now what he COULD do is have an honest conversation with Seugjoo confronting the risks of a relationship and what they want from each other and how to move forward. Obviously not as dramatic of a story lol but it would be interesting and sweet! I’ve seen it happen in certain mangas before and it should totally happen here.
THIS
Muheun's hot and cold attitude is due a lot to trying to put up boundaries with someone he is interested in him but who is too young for him. Seugjoo keeps pushing past those boundaries and initiating intimacy and that's the only way for them to have a relationship. Honestly what is the non-asshole thing for him to do here? To romantically pursue his male little cousin who he knew as a toddler? To tell this kid that he wants to be in a relationship with him when he's a teenager but insist they don't do anything physical?
When Seugjoo comes over to his apartment in the middle of the night, to scream at him until he leaves? To treat him coldly and insist on those boundaries that Seugjoo keeps blowing past? To start taking him out on dates sincerely acting relationshippy with someone he feels is a child? There is no way for Muheun to act here where he comes out of this not the asshole. The younger person has to be controlling the momentum of the relationship.
And it's not like Seugjoo has ever asked Muheun out on a date or anything like that either. Kissing someone while drunk and then expecting that they romance you in response while you ignore what happened is also weird.
Neither of them are handling this maturely. We expect Muheun to be mature because he was older. But if he got serious about romancing Seugjoo it would just be creepy due to the age difference.