@Ayo he's not physically cheating, but he is blatantly flirting with another man which can be seen as unfaithful. Sun Yool is doing something similar with Yechan, it's less obvious - but they have kind of been flirting. They both know their relationship is coming to an end, so they don't really care what happens next. I think it shows them giving up on each other 
I'm quite straightforward & tend to answer questions if i see them. 
Please forgive me if i come across as blunt and/or take you too seriously.
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