How long did they date before ch 78?

Not the Chunslut T_T My messed up boy.... I get it though.

It's not that type of cheating. It was the culmination of regrets, frustraion, self-sabotaging and letting go. They actually hadn't been dating exclusively up until that point. They had just gotten back together after breaking things off and spending time apart, Chunwoo actually made the first move to get back together, but he STILL hadn't ended things with Taesoo or decided to be committed to Ho-In.
Chunwoo felt it was unfair and was also punishing himself for holding on so long to an undefined, co-dependent relationship which he hoped would one day become official, but eventually knew would never happen, so letting go meant there was nothing to show for all that time, pain and persistence. He also felt guilty that his childhood friend would be alone once he cut off that part of their relationship.

But starting things back up with Ho-In was Chunwoo's way of saying "this is the final stretch, I WILL break things off, it's just a matter of a time". Which was still selfish, but a step further than before. And then RIGHT before the cheating, he made a promise to Ho-In, that day he'd be choosing him over Taesoo, implying they'd also be exclusive from then on (with the understanding that Chunwoo's feelings for the "ex" would still take some time to get over from). And that's when Chunwoo fucked up, lost his reasoning amidst emotional turmoil and jumped Taesoo. It wasn't love or lust, it was pain and violence. Which sucked for everyone.

But yeah, for me, the fact the he regretted it genuinely right then, cut things off and ran to Ho-In to take responsibility did make me feel it was possible that they could eventually recover from this, IF Ho-In wanted him back and Chunwoo did everything in his power to actively make up for the trust he destroyed and the hurt he caused.