Spoiler
I'm not sure if it is poly, however I doubt it will be. In the next chapter we see that Johan (MC) was catching feelings for Jinyeong(right guy) but the mom is against the relationship. So one day Jihan gets a text to go to a hotel and he runs into Jinyeong there. However, Jinyeong is with a girl and has his arm around her, so it seems he is dating her. But then when MC tries to talk to him, Jinyeong blows him off. They aren't the same after that. So it seems Jinyeong hurt MC in a way that was similar to what his ex did and that is why he doesn't not want to be in love with Jinyeong and draws a line now. Also the season ends with Jihan running into his first love.
Wait a min. If their relationship was supposed to just be fuck buddies from the beginning then why tf is he saying “I love you” and no line was drawn. I can see why he thinks he has a chance because he was being dragged along all this time