I just Marathon a drama called "BAD LOVE". Please watch it. The drama will be great for mature audience watching. Drama deals with affair and it's consequences. For me this drama is underrated!!! It should have high ratings as the plot of the story, and acting deserves it. The main leads did a great job!.

What I loved about his drama is the fact that you knew affair is wrong, but in this case you really could not take sides here. No matter what you decided and which side you would go with, there was no right or wrong answer to it. 

I cried, think a lot and rethink a lot while watching this series. I highly recommend it.

Honestly, this is one of those series that you will remember. Till now, till the end I would say - this drama shall be on the list of dramas to watch and learn from.

Give it a try! 

Well, the cast is great but I honestly wouldn't be able to sit through 20 hours of people cheating/having affairs; to me it's like one step further than love triangles. 
It actually didn't have much of a following back in the day since cheating is very frowned upon (especially if it's from the woman's side) and I know a lot of people here don't enjoy these dramas but I'm glad you liked it so much!

PS: If you enjoy affair dramas then you should check out Valid LoveQueen of Ambition and Secret Love Affair.
Ireland
Hiraguo - two married woman who become friends and one kind of drags the other into having an affair. two different side since one has children and the other is having an affair with a married man
What Happened In Bali
Beloved - although I didn't see it yet
Majo no Jouken - she has a fiancée, but it's more about the taboo of student x teacher and the age difference
Que Sera Sera - not at first, but they both get married
Sora wa, Totsuzen, Arashi no You ni
Doctor's Affair - the side characters
Please Come Back, Soon Ae
Temptation - with the same lead as in Bad Love (Kwon Sang Woo), didn't watch yet but I didn't hear great things about it (compared to Stairway to Heaven where the two mains played a long long long time ago and was great)
90 Days, Time For Love - also didn't see yet
Lovers (2006)- kind of because he has a gf who's pregnant (but it's a really funny drama too, one of my older favs)

Hmmm I see I have a thing for that...
Hmm.... how about  Naked Kitchen? It's a movie not drama, but it is rather unique in how it ends. It's also quirky and as a whole it reminds me of  french cinema in a way.

Also, seconding Secret Love Affair

@EvilYuli Temptation fascinates me mostly because of the whole backlash it gets