There was this game where the other guy asks MC to let him borrow his wife for a weekend and pretend to be his fiancee because he lied to his parents. Then a weekend turns into a full week, they sleep together in a room and over time they get closer. They whisper each other away from the MC.
Its basically a cuckold half way because the MC already have these thoughts that his wife is cheating. Its the subtle cuckold/cheating that was so hot.