
Honestly they really do not seem good for each other. I see a lot of ppl ganging up on kanghee for his insecurity and while I acknowledge seongjae's growth as a character ultimately his actions are what caused this insecurity in kanghee to manifest. Don't get me wrong I'm not huge fan of kanghee but I definitely see how and why his insecurity has been festering. Even from their 1st intro as a couple u see seongjae kissing kanghee but staring at Dongha. It has also been hinted that seongjae was only interested in kanghee cuz he looks like Dongha and kanghee is aware of this. Which definitely could cause kanghee to think that he is only be used as a fill in for Dongha. Not to mention they would break up multiple times & in some of those times seongjae would hook up with someone else. So from kanghee pov it kind of encourages the thought that he was just another notch in seongjae's belt cuz he couldn't have dongha. Now to the "cheating" claim. I don't believe kanghee intended to cheat from the chapter it looks like he really just wanted to go to the beach with someone cuz well he doesn't really have anyone to go with. Seongjae said no & it looks like he doesn't have much friends, it also looked like he was about to clear the misunderstanding when seongjae stepped in. They don't seem to have a great relationship overall and personally I think it was cuz of timing. Maybe if they had met much later with seongjae being completely over Dongha they would have had a chance but as u can see from the beginning of their relationship seongjae was still in love with Dongha & it was obvious to kanghee. I honestly don't know if its even possible to mend this relationship
Although it was rushed I still really loved the ending, I hope they do a spin off of the red head and the lil bro