ehem the main reason for polygamie was to avoid poverty when a women is alone because her husband died -->function
we could make something similar
like for every wife+ you lose a higher percentage of your experience if you die the boon would be smaller...I thought about 10% more loss on exp. for every women
and then I would include a higher "bride-price" like
1: first women as it was
2: (1 + following) 25 gems for the festival and to show that he is rich enough
3: (2 + following) *even higher price use your imagination*