To the person that didn't see the show but decided ro make a review, neither of the women are the cheaters. The man is, and he isn't looked upon favorably for it. The mistress is also not one we're meant to believe is in the right. The media often sensationalizes cheating but I think its also worth noting that more often than not, cheating even in media never leads to anything good.
Now, as a review for the show, I started watching as a fan of jeon so-min, and man is her character so much different than how she usually is. This definitely is not my usual style of drama (think Penthouse vibes: wealth, cheating, scandal after scandal). However, I still enjoyed it well enough and I can recognize that it's a good show. Great cinematography, complex characters, and lots of conflicting feelings.