Why he storming out

That's understandable he's only 23 years old and not that fully mature when it comes to relationship. I was also acting that way when I was that age but as you grow older you'll realize how childish those actions are but gain on the other hand is already 30+ he's fully mature in handling things and in making decisions. He didn't abort the child right away he carefully weights things before he made the decision for both of them. He consider both his and his lover's current life standing before he made that decision of keeping the baby and I applaud him for that.

Actually he is 26 so he should mature enough to be wise in such situations.. But he just had a tantrum Like a five year old and left the house.. So although I love him but to be honest he is pretty childish and immature.

To be fair that's his Korean age so his international age is more like 24/25. But I honestly think that he is right about getting mad. Imagine wanting to know everything about your partner but the other doesn't even seem the slight bit interested in you. Gain, yes he loves him, but also hasn't been in a relationship and he has the power to find out everything about Hyeon-oh without asking him. He doesn't tell him anything, so of course when you're having to find things out by someone else is gonna hurt. These two need to talk and get their stuff settled.
Don't get me wrong I do think that Hyeon-oh needs to see Gain's perspective but him storming out was a bit over the top for me but him getting mad is understandable.

When he talked about the parents, that kinda pissed me off bc he met gains parents and all and has the audacity to say that about his side of the family.