I think the author meant to say that, when the Dungeons started to appear, it somehow affected the birth rate of Females and increased it to "1 : 5" (for comparison, current global male & female gender ratio by birthrate is something like "1.05 : 1").
It just meant that, for every 100 girls born, only 20 boys are born in comparison.
It didn't say that the death rate of men increased, as both men & women die with equal chance inside a dungeon.