Secret Love Affair is really good- FL has an affair with her student. Husband sucks so there is no need to feel bad for him.
A Man and A Woman- FL and ML have difficulties with their kids and meet because of it but both are married (movie)
Anata no Koto wa Sore Hodo- The only people not having an affair if, I remember correctly, are the people getting cheated on.
+1 Misty- I highly recommend this one
Boku no Yabai Tsuma- The ML cheats on his wife and the story takes off from there
Hirugao- Bored housewives cheating on their husbands. One has been doing it for a while and she teachers another who is a newbie.
Mistress- One of the ladies cheats on her hubby while another lady sleeps with a married man. Not really the main focus of the story but it does play a part.