If you really understand the whole entire story, it isnt about cheating. It is actually about the challenge faced by marriage couple - Trust.
Seok-hoon have no intention to cheat on his wife Hong-joo but she kept driving him nuts and pushing him off cliff for him to admit that he had feelings for Se-young. Seok-hoon in fact control it very hard not to fall in love with such charismatic Se-young and Se-young too is avoiding Seok-hoon. But Hong-joo kept driving him away.
Hong-joo insanity and forceful character is very frustrating! She played so well that I hated her insecurity so much! HAHA! Is that a bad thing?
In this whole story i quite like Min-woo, he's the type where he's fearfully admit his love and willing to do anything to get it. I'm rooting for MW and HJ but sadly they just didnt work out.