I consider that the biggest plot hole of this story, tbh. Nanjo is supposed to be rich and powerful, but just about anyone can swindle him?
His deal with Mokyeon is also bonkers. He has a boyfriend who just won't stay loyal to him, and instead of bringing it up with him, he goes and demand the lover to back off (and okay, I can kinda understand the logic here, many people do that).
But when the lover proposes sleeping with him to leave his boo alone... He accepts? This is unhinged!
If he's such a rich and powerful person, he could afford to get rid of him, through either legal or illegal means? Why give up his ass for him?
We're giving the author too much credit, I consider this a plot hole.
I don't see a plausible reason why Nanjo accepted. If that's ever addressed, I suspect the author is going to hit us with some "Nanjo unconsciously wanted Mokyeon" or "Nanjo though he could learn from Mokyeon" bs.
The only half-decent explanation I can think of is if Nanjo actually planned to have Mokyeon and Sunhwa have this big fight, and expected to be able to convince Sunhwa afterwards.

You guys keep on babying Nanjo, it's ridiculous. Every criticism made to Mokyeon's obsession with Nanjo can also be made to Nanjo's obsession with Sunhwa.
Mokyeon should leave Nanjo alone bc he doesn't want him - Nanjo also should leave Sunhwa alone bc he doesn't want him.
Nanjo has every right to be mad at Mokyeon for lying to him - Sunhwa also has every right to be mad at Nanjo for lying to him.
But Sunhwa cheated on Nanjo - and what exactly was Nanjo doing with Mokyeon? It's not normal to accept to sleep with someone else so they stay away from your partner. Nanjo did that because he wanted to. There wasn't some powerful coercion, he accepted those terms.
Nanjo is not some poor thing, everything he's going through is of his own doing. He's the all powerful and rich CEO of a company, it's unbelievable how gullible and naive he also is to believe Sunhwa blindly, and to accept Mokyeon's proposal.
He did all that because he wanted to. He's not a poor baby.
At the end of the day, Sunhwa is the most sane character here. I'll defend him to the end.