I don't think Nanjo was ever going to let go of Sunhwa for Mokyeon, like that was somehow the sole reason he was still sleeping with Mokyeon. He wanted to be Sunhwa's "saviour" so bad (which was embarrassing to read), and he didn't want someone like Mokyeon "harming" Sunhwa.
I think that’s just a surface explanation he was giving himself, I think he was in a very confused headspace.
Sunhwa even said that things weren’t going well between them and that Nanjo wasn’t even able to get aroused without Mokyeon present and he was also annoyed at the fact that Mokyeon slept with Sunhwa.
Mokyeon fumbled Nanjo so hard. They were getting so close, he was enjoying the sex and they were even texting, he should have just been patient , asked him out on another proper date and not pull that shenanigan he did at the end of the first season.
He forgot that Nanjo has just been discovering his queer side, it’s one thing to be the seme (easier to cope with in a heterosexual lense) and it’s another to be the one on the bottom (much harder to accept for a man who viewed himself as straight), Nanjo clearly struggled with that but he was coming to terms with it.