It was literally an arranged marriage, so why are you acting like he was in love with the original Hae-soo? You should actually read the spoilers and understand the whole context before judging the cheating plot.
I hate cheating too, but context matters here. The brother was in love with the original Hae-soo, and from what I read in the spoilers, Hae-soo also had a lover. The ML knew about everything from the beginning and genuinely didn’t care because he had no feelings for Hae-soo. In fact, he was deliberately letting it happen so he could use it as proof of infidelity and file for divorce. It wasn’t some situation where he was clueless and got betrayed.
Even after Doo-sik exchanged souls with him and entered Hae-soo’s body, the ML still didn’t fall in love with him immediately. His feelings developed much later, around the middle/end of the novel. So I really don’t understand why people are treating this like some typical cheating trope. It was a loveless arranged marriage, the ML knew exactly what was happening, and he was completely fine with it. The context matters.
from what I understand, the ML didn’t even sleep with the original Hae-soo in the first place. He was cold toward him and clearly didn’t care about him as a spouse, so it’s not surprising that the marriage turned out this way.
And why are you putting all the blame on the omega? The brother-in-law was the one who came onto Doo-shik in the first place; it wasn’t Doo-shik actively pursuing him. The situation was also happening within the family dynamics, while the actual husband already knew about it and didn’t give a shit because he had no feelings for his spouse anyway.
I’m not saying you have to like the setup I literally hate cheating plots too. I’m just saying it’s not the same as an ML being genuinely in love with his wife, getting betrayed behind his back, and then finding out. The context is completely different. If this kind of relationship dynamic still grosses you out, then yeah, fair enough, it’s just not your cup of tea.

I don't know what is interesting about cheating situation here lol, maybe it's not a cup of tea of mine, like cheating shit is normal and this is why I hate Omega's who got easily touch or sleep with other alpha like it's nothing. I just see it disgusting ain't fun of it.
I hope it's not happy ending and they all suffer till the end.