I was thinking a bit about this world where there's 1 man for every 5 women. If you think about repopulation, this would mean that each woman would need to have 6 children to maintain the population. In this case, there would have to be at least 1 son and 5 daughters per woman. Either that, or each man would need to have more than one wife to keep the population stable.
But the manga itself says that polygamy is forbidden. So, the most likely scenario is that each woman has an average of 6 children. Or perhaps there are many cases of "clandestine" polygamy. Lastly, another possibility is that this difference in the number of men and women is a recent development in the history of this alternate world.