The mistake you are making is to compare Feudalism of this game to Feudalism of Western Europe. While there are strong cultural similarities with Western Feudalism, the matter is that the Dev is a Russian and in Eastern Europe Feudalism operated in entirely different manner.
And how they differ? In Kievan Rus' (ancestral Russian State) the ruling King was a powerful warlord who pretty much did anything he wanted. Rurik created Kievan Rus', Igor raided Constantinople, Olga massacred Drevlians, Sviatoslav invaded Constantinople again, Vladimir Christianised Rus' despite having something like 7 wives, and Yaroslav enjoyed the Golden Age of his people. Then things fell apart.
And mind you, this game is far diverse than both Western and Eastern Europe ever were. There are elves, Republican western cities, Scandinavian north, and Arabic south (plus east which I won't remember). If you become the gigachad and control all these diverse lands, having extra wives should be no problem.
This is how you learn history on a decadent porn forum guys!