There's an historical accuracy as Napoleon did divorce "in the interest of the Nation". He needed a heir, and with Josephine, they weren't able to have children. That's the original letter sent to his cousin, summoned him to secretly come in order to process the divorce (from wikipedia - Napoleon and Josephine divorce):
La politique de ma monarchie, l'intérêt et le besoin de mes peuples, qui ont constamment guidé toutes mes actions, veulent qu'après mois je laisse à des enfants, héritiers de mon amour pour mes peuples, ce trône où la Providence m'a placé. Cependant, depuis plusieurs années, j'ai perdu l'espérance d'avoir des enfants de mon mariage avec ma bien-aimée épouse l'impératrice Joséphine; c'est ce qui me porte à sacrifier les plus douces affections de mon cœur, à n'écouter que le bien de l'État, et à vouloir la dissolution de notre mariage.
Parvenu à l'âge de quarante ans, je puis concevoir l'espérance de vivre assez pour élever dans mon esprit et dans ma pensée les enfants qu'il plaira à la Providence de me donner.
Translated:
The policy of my monarchy, the interest and the need of my people, which have constantly guided all my actions, require that after months I leave to children, heirs of my love for my people, this throne where Providence placed. However, for several years, I have lost hope of having children from my marriage to my beloved wife the Empress Joséphine; this is what leads me to sacrifice the sweetest affections of my heart, to listen only to the good of the State, and to want the dissolution of our marriage. Having reached the age of forty, I can conceive the hope of living long enough to raise in my mind and in my thoughts the children that Providence will be pleased to give me.
As for the norm, it was Napoleon himself who declared polygamy outlaw in 1804.
L’article 147 du Code civil
Selon l’article 147 du Code civil en France : « On ne peut contracter un second mariage avant la dissolution du premier ».
Translated:
According to article 147 of the Civil Code in France: “A second marriage cannot be entered into before the dissolution of the first.”
Polygamy is illegal according to French laws because it creates a difference between persons.
such a marriage would conflict with the French conception of public order in international law in that it would be an attack on equality between spouses, which has constitutional value in France.