I though that there would have been some kind of a misunderstanding but nope. It's a cheating alright, and…
You might not understand but the drama is actually pretty accurate. In reality, cheating is an irrational thing. Most of the time there's no reason for it. And if there was it's probably just used as an excuse to justify what they have done. It's just humans being irrational and selfish.
Omg you have to watch "What Happens to My Family?" I fell in love with Hyunjoo/ Do Hae Kang because of that drama. 50 eps but I finished it in a week hahaha.