Duh 说:
Game is a game. History is history. Gameplay always trumps realism. Same reason why you don't have to take bathroom breaks
a bunch of work.
... why bother including the feature at all?
Sorry to chop up your post like this; I'm not ignoring the rest of it, but I want to address these three points directly.
Firstly, I totally agree about gameplay trumping realism every time, and I accept that certain liberties are taken with historical accuracy for this purpose. I also accept that including polygamous marriage would probably involve more developer work than monogamous marriage. However, I don't think it'd be beyond the competence of the clever folks at Taleworlds and I do think it could well be worth their time and effort to include it. I also don't think bathroom breaks are a good comparison.
You don't have to tell your character to go to the toilet because it's not relevant to the focus of the game; nor do you have to sleep or eat, or even rest your troops on a long march. But the game does include marriage, because that is relevant to the gameplay (strategic and RPG elements) and to player immersion in the semi-historical scenario they are trying to create. Marriage is not a universal human need (like emptying your bowel regularly), it's an aspect of culture and society. Bannerlord will include a variety of different cultural groups, whose historical counterparts did have a variety of distinctive marriage customs. Having different cultural behaviour would go a long way towards making Bannerlord's factions feel more distinct from one another, beyond merely what type of armour their soldiers wear or the architectural style of their towns.
Apart from that, it could add depth and strategic options to the gameplay. The most obvious concerns diplomacy: more potential marriages mean more potential for blood-alliances (family to family, rather than necessarily kingdom to kingdom). And the player would have to choose whether or not to marry into a culture or faction that allowed polygamy. Perhaps having several spouses would damage your chances of forming good relations with lords from factions that frowned on the practice? Maybe it would be a way of gaining influence points with certain lords (and losing them with others). It also would extend the courtship mechanics beyond the player's first marriage, keeping them relevant to the gameplay throughout your campaign.
Whatever the practicalities of implementing such a system, Bannerlord wouldn't be the first medieval-themed game to feature polygamy as a possibility for both the player and the AI. And it has been in development for a long time - as the sequel to Warband, which includes marriage and personal relationship building as a key part of the single-player game, I would expect Bannerlord to have been designed to improve and expand on all the gameplay aspects of its predecessor from a very early stage.