Jeonghan knew it. He knew that Seungcheol was cheating on him. He didn't want to believe it until he walked into the same restaurant that Seungcheol and his "boyfriend" were having a date in. 

This story can go 2 ways:

1) make it really angst and he is cheating 

2) if you don't like breaking them up, then it can be a misunderstanding.