Hey, polyandry is a thing too. 
Seriously though, I'd like to wait for Anaris's input before we expand the feature complication with socially progressive concepts in a medieval-based game, even if we're enlightened players and the game isn't meant to be an exact simulation.
Polyandry is a form of polygamy. Poly (many) gamos (marriage). Polyandry and polygyny (I totally know what to do when I have too many Ys in scrabble now) and its declinations.
I'm not sure those should be viewed as "socially progressive concepts", but rather, "foreignly-sourced concepts". If you look at the map on the wiki for polygamy, it's basically everywhere except in Europe and its ex-colonies. And still is. I don't see many social justice warriors fighting for legalization of polygyny...
I think it would allow for more roleplay opportunities. We aren't talking about having the game start using 100 different genders, here... And it's not really a modern thorny issue, at least not in western society, I don't see it spawning a ton of hateful forum posts.
Not sure about same sex marriage, though. In-game, that is. There are some people that are sensitive to that IRL (mind you, I'm not typically a fan of tending to every sensitivity). Where to draw the line? It could probably be argued that some cultures allowed it, with the same arguments as for polygamy. I don't know. Doesn't feel the same to me. Harems and such are common pieces of fantasy, and were present on the very border of Europe, with nations that Europe actively fought. I'm no homosexuality historian, but from the little I know, the places that accepted it were much farther away. Doesn't lend itself to that "close foreigner" feel. Also seems much more susceptible to troll baiting. And on the other hand, forcing gay characters to marry a character of the opposite sex purely to legalize their union seems like it could lend itself to legit interesting roleplay situations.
In any case, I think both of these concepts are intimately linked, no matter what one decides on either, because polygamy in a binary sex system necessarily involves the union of at least two people of the same sex.
