This implies that Jh had the ability to have that conversation in an informed, autonomous way. Garam is the first man Jh has EVER dated and they started out as fuckbuddies and Jh keeping Garam his dirty little secret. It's totally fair that Garam wouldn't have the conversation right away until they both got their footing. And Garam DID let Jh know that he wasn't out and that he had trauma surrounding that. They needed to sit down and have a conversation, for sure, but neither of them initiated that and both just assumed that their own desires were the default way to go.
Jh needs to experience coming out without a partner connected to that identity to know if coming out is something he genuinely want because he feels safe and proud of his sexuality and not because he's stubborn, has a very hetero-normative view on relationships, and wants to show off Garam as his bf against Garam's will.
To add to what you are saying, in the conversation with Jaehyuk's friends about living with Garam outside of the dorms, it got casually brought up that Jaehyuk referred to Garam as a "girlfriend" to them. This shows that Jaehyuk himself knew damn well his friends wouldn't accept him having a gay relationship partner because you don't hide something without reason. This was direct confirmation to Garam, in that moment, that redhead and hat guy are not allies and therefore are not safe to come out to. It would be out of character if Garam didn't freak out over Jaehyuk, throwing caution to the wind, to out them to the same people he lied about having a "girlfriend" to. The same people who are shown to be horrible womanizers who are openly comfortable with cheating on women, and even go as far as trying to force their friends to cheat too. Jaehyuk told them about his relationship problems, and their solution was for him to have s-x with the women in the bar, even despite Jaehyuk saying his relationship was still standing. They completely disregarded his relationship as nothing and disrespected him more, using his bathroom break as a green light to invite women to their table, even though he clearly stated he was against it. The redhead then answers a phone that isn't his and tells a person he doesn't recognize (Garam), who could have known his "girlfriend," that Jaehyuk was there to pick up chicks". If you think you are convinced you are answering the phone to a woman, and a guy answers instead, wouldn't your first thought be that this person must be her friend or a family member? So don't you think it would be unbelievably sh-tty to tell that person that he is actively cheating, when he's not. Garam probably hasn't forgotten the conversation with the redhead that night, and definitely understands the type of person he is, because if I can connect the dots, so can Garam. Hat guy and readhead are the least trustworthy people to out yourself to from Garam's pov and Jaehyuk was about to out them without a single care in the world.
I'm so over this you can't date in secret, you can't hide forever bullshit. I'm from a conservative country as well. I have met people who have hidden their entire lives. You can't just spring up that your boyfriend is gay in public without any talk. That's just violating boundaries. It literally changes the course of your life. It's so ignorant take that Garam should have let him cause "you can't hide forever" "Jh loves him" "it's the least he could do".
Y'all call running away immature but the reason he ran away was his bf just disregarded a major boundary. But we can put a blind eye to that or plast a "cute" reasoning to it.
Also Jh knows Garam is in closet. He doesn't know the trauma that surrounds it but he knows it. During their arguments, Jh explicitly says "I know you are soft about these topics" or something like that. So it is disclosed. Plus during their first fight, Jh addressed Garam as his "secret girlfriend" that also implies that he is aware of Garam's preference. If he was truly dumb on these things, he would not have bothered to address Garam as such.
Again coming out is something you talk with your partner on a regular tension and argument free day. Not be like "oh today I feel like doing it so I'm gonna do it. fuck my partner's feelings. I'm gonna stay ignorant". The words are all over the chapters, readers just tend to ignore it and excuse it.

If you want to live your entire life hidden, maybe speak up first during the start of the relationship? This is one of those things you need to really clarify and let your partner know.
The fact that he chooses to run away instead of breaking up due to what's essentially a relationship dealbreaker is extremely immature.
I say this as a gay person in an extremely homophobic country, you cannot seriously expect to date in secret, that's not how life works. Hookups? Maybe. But serious dating is not something you can hide. Garam lost any sort of high ground the moment he gave in and became his boyfriend. What, did he expect to live under a rock the rest of his life?