
It's not "entirely coerced" as NanJo accepted the proposal. It's more the act of doing it and then keep being strung along because he thinks he's doing the "right thing" by protecting SeonHwa from MokYeon, not knowing SeonHwa is the one behind the threesome and not MokYeon. So he thinks MokYeon has the bad guy and bad influence on SeonHwa. He treats MokYeon like trash. Which is unfair to him. Despite his obsession, he still isn't the "bad" guy that NanJo thinks he is.

Look I get you but 'not entirely coerced ' is still coerced. Nanjo accepted it with no full information of the fact that he's being used, while MokYeon is aware—Mokyeon is in a privileged position here, he uses Nanjo's nescience to take advantage of him. As far as Nanjo knows, he's doing the right thing, and he has no other choice but to do this right thing in spite of the fact that he does not like MokYeon.
Consent must not be coerced and it must be informed. You can argue that Nanjo is stupid for not seeing through his situation, but does no change the fact that he's not informed (hence it is the job of the other person to disclose appropriate information)
I'm not saying there SHOULD be consent, I know this is fiction and things are done for the sake of plot, but we must also accept that what ML did is still coerced, and would still be unacceptable IRL.
Thank you for listening to my TedTalk ╥﹏╥

Not coercion but deception. He was NOt coerced. Nobody tied his hands behind is back. Nobody "threatened" him. He "feels that MokYeon is a bad influence". If you say MokYeon is bad, so is SeonHwa. He manipulated NanJo into thinking he's this pure innocent soul when he was the one who had orchestrated the WHOLE threesome with sadism. The only one fooled is NanJo. While what MokYeon does is despicable, the one that set this whole thing up is SeonHwa, and with his carefully crafted image leaves NO ROOM of doubt in NanJo's mind that MokYeon is the pure devil that has lurer his innocent SeonHwa in. NanJo, at ANY POINT IN TIME, CAN ABORT this mission. It's manipulation and deception, not coercion.

You really need to reevaluate how you define consent, this is concerning. My reply to your other comment extends to this. Unless you claim to have a more philosophically rigorous alternative to the concept of consent, your points here are invalid. I will not argue on that further.
What I can add is if it would've been consent if MokYeon told him that SeonHwa is deceiving him, before offering the agreement to have sex. This clearly did not happen.
Agreement born out of, what you yourself call 'manipulation and deception' is not consent, my goodness.
This is interesting in a disturbing way if y'all get what I mean