If you don't mind movies there is Obsessed a Korean movie a couple has an affair cheating on their marriage partners, plenty of skinship(it's erotic)
When A Man's in Love - a kdrama the FL doesn't really love her fiance and almost cheats on him, constantly hides from him with another guy. Pretty sure there is a bit of an age gap, ML is way older.
Discovery of Romance - a kdrama Nobody openly cheats on anybody, but the girl has a difficult time deciding which guy she likes while she is already in a relationship, I suppose you can call it emotional cheating.
