His father and him got into a huge fight. After lying for years, thinking it's for the better, they didn't realize that the one that ruined his life was them. His own family. If you realize your mistake, shouldn't you own up to it?
His grandpa forgave his father despite finding out that Yoonsung's father was just manipulating his grandfather, that reason is enough for his father to forgive Yoonsung's rebellious act, guilt can kill y'know? And what I meant for the rebellious act is that.. for them, they hated Hakyung because he was gay and he liked Yoonsung.
They also gaslight themselves that Hakyung was the problem. When in fact, Yoonsung fell for Hakyung first and despite knowing he was a man, rather of fearing and feel disgusted, he felt fear that he might lose Hakyung because he found out his secret. And about Yoonsung falling in love, I think it was mentioned already. Hakyung was unique, out of fake people, Hakyung was different, not only to Yoonsung but to his grandfather also.
Some people fall in love without any reason, that includes me lol. It's a true thing that happens irl. But most of the time, we like a person because they're different.
That's all ^^
I get the logic behind the decision, after all this is a romance story and the main couple should end up together so the family would have to realize their mistake. I just think the execution could be better, a few more panels to justify the conclusion instead of relying on the readers to fill the blanks. I'm not only interested in the result, but also how they got there. It was resolved in barely 3 chapters and I don't think it's a very satisfying way to tie up a problem that took 70+ chapters.

Maybe I went with too high of expectation and didn't have the right mood when reading this but I didn't feel the intense emotions that others did.
To start, I will give this 5/5. It's a solid read overall. But the characters are always drawn so pretty and perfectly, that I failed to immerse in their emotions sometimes. With this kind of stories it's important to be able to convey nuanced and complex expressions, but many times I feel like the intended feelings didn't get conveyed properly and the expressions were stiff and bland. Like when Yoonsung got slapped, it should have been the climax moment when the family understood he's been hurting, but his expressions didn't convey that great of emotional pain. Or when the SIL finally apologized to Hakyung, I didn't feel satisfied because it almost felt like a half-hearted apology.
The conflict resolution also felt very anticlimactic because of it. Like they lied to ML for years, to the point of fabricating a newspaper article, then everything gets resolved because the mother suddenly throw a tantrum on a random dinner? How did that came from, when she almost never had an interaction with her son? And then suddenly the father was okay with it too the moment he saw the video from grandpa? And what happened with Gijun's sister? We never saw her again after she got drunk that one time.
I also wish we know what Yoonsung was thinking beyond just the flashbacks, like give us his inner voice! We know Hakyung's inner turmoil and conflicting feelings, but we know almost nothing about YS other than he's this greenflag yearning guy who loves HK. What made him fell in love? What did he like from HK?
Also +1 to the other comment who said there were conveniently too many accidents in this comic