
I think Jiho shouldn't be entirely blamed here,of course,he made a mistake and he deserved what he suffered afterwards.But he is also someone who endured it for years.He waited for Younji to tell him but Younji didn't (of course,he had his own reasons and own sense of insecurities)He himself has strong feelings for Younji and he himself was in a fix.He thought as a close friend it's his duty to bring them together,on the other hand he was jealous.It would be weird if he could have controlled his urge to mention the whole thing especially after the heated argument with this hyung.
What Jiho feels for Younji isn't clear,it can be romantic love or love between close friends.It's just a critical situation for anyone who wants his beloved to be happy but at the same time finds out that his beloved's happiness is with someone else and that someone is his brother.
The problem is he acted without thinking,he lacked maturity but then again it wasn't entirely his fault.The situation was difficult.

And the problem isn't outing.even if Younji were in love with let's say his older sister the problem would be still the same, others are not supposed to interfere here.
Younji couldn't tell fearing the relationships wouldn't be the same after the confession.When Jiho casually said everything it was like what he had been treasuring for so long came into pieces,his sufferings seemed to be useless.
Why Younji decided not to tell was for many reasons included the fact that he's a boy,But the reason he got angry isn't because he got outed but because he didn't wish Jiho to know.
Wow!that's the most adorable proposal I have ever seen!