What i've hinted at earlier in the story when namjoon said his parents sent him to the house for being with jin is-

Namjoon is straight. He's just a straight man who ended up loving a guy.
Things like that happen, but his parents didn't like it.

Jin is his one and only MALE partner.
That's also why the only real sexual interactions he's had with anyone else in the story only happened when he was with jin (the threesome).

He said his parents set them up, he never said he didn't at least LIKE her——- i mean, why would he have the letters still if he didn't feel for her at least a bit?

Cheating is still cheating. If he had the freedom to break it off 12 months later, that means he probably had the freedom to break it off anytime before.

Sorry to be PRESSED but like  its moreso at myself than towards anyone reading. idk i felt like as an author i did a shitty job getting that across. I'm sorry