Comedy?

IMDB lists this as a comedy. A comedy, as far as I know, is a genre of film in which the main emphasis is on humour. These films are designed to entertain the audience through amusement, and most often work by exaggerating characteristics of real life for humorous effect and traditionally have a happy ending. Even in a more dramatic work in which the central character is faced with an adverse circumstance, the result has to be a successful or happy conclusion.

I find nothing humorous about either the subject matter or the main characters. What humor was there in the depiction of the cheating wife who is tempted into having an affair for no other reason than still having an infatuation for a previous lover. There was absolutely nothing lighthearted or funny about the way she behaved. As far as a happy ending, what was there to be happy about? What is humorous about a wife who has to live the rest of her life in a lie and keep her husband in the dark about her affair? Are we supposed to accept this as a comedy because infidelity is just something trivial or because she was successful in not having to reveal her infidelity but live with the fear of someday being found out?