
I read a lot of science fiction. Even though they have a lot of elements which are out of this world, they do something called as world building. Setting up the rules and culture for that world. If it's not done then we can assume real world rules apply. That's why in the first sentence of OP I mentioned about world building.
And what's wrong with looking for some sort of common sense in fiction, it's still written by real people. And the genre is definitely not "crack", so... yeah I still look for logic unless otherwise mentioned in world building.

You are so right about everything you mentioned, I totally agree with you. I will add, It was so cringe when Lia was surprised that Jae got darker due to the sun as if tanning is something horrendous. These characters also look nothing ethnically Korean, they are racially ambiguous leaning towards Europeans or mixed with Europeans. What will a manwha be without the white saviour aka Matthew lol. I’m mixed also so I don’t mind Matthew and his omega but both their characters didn’t add much to the story except their children being partners for the other couple children. Seems like the artist/author have some kind of fetish for Europeans lol.
All the omegas expect the second couple are blond. All the male alphas except the second couple are dark haired. Like how. Like why. If this is the type of world you are building may be start with blond represents omega and dark hair represents alpha. Would have made more sense than this.
2 brothers are in relationship with a different pair of brothers. Each pair of brothers has an uke and seme (omega and alpha).
Joo brothers - Joo Ian has blond hair and Joo Jihwan has black hair
Ro brothers - Ro Jaeho have dark hair and Ro Miho has blond hair
Joo Ian is omega/uke and Ro Miho is omega/uke. Both blond hair. Being the uke.
My understanding of genetics says that brothers would not have such complete 180/ opposite hair colors. Ok assume that due to some rare strong genes, one of them might have inherited a different color. How in the hell did only the omega/ukes inherit this trait? And not just one but both the omegas from the 2 pairs of brothers did. Where is the logic in this?
Coming to the children.... Stupidity ensues
Why is the blond twin an omega? The Dark Haired twin is the alpha and of course his omega partner is blond. And the blond twin would obviously be paired with the dark haired alpha (Noah?). Just stupid racial domination or fetishism thing these authors seem to have. Uke almost always needs to have lighter hair than seme.
Also are there no other families or no other alpha or omega in this? Why the hell do even the child get together only between these 2 families. Extreme fantasy with no sense.