The main reason why polygamy was justifiable in the olden days was because there used to be a lot of wars over territories and a single war could see thousands of men dying in one day, resulting in an excess of women. Women generally did not take part in wars and the number of women on your land determines your population growth and that is why Osun is a woman. Later, men started to also fight wars of theology in addition to war over territory and even more men died. On one occasion Mohammed was said to have personally beheaded over 800 war captives, I think for breaching a treaty or so. All were beheaded one after the other by him. Such things lead to justifiable polygamy. In some pre industrial west African communities that were peaceful and in which big men still wanted to be practicing polygamy despite the peace, they had a public house of community sponsored comfort women which any unmarried young man can go to, to relieve his urges at majority community expense. Now there are no wars over territories in most part of the world (which is why we have development dichotomy) and there is no longer a shortage of men as it used to be in the relatively more unstable pre industrial society

There are many women in modern and ancient times who have the means to marry more than one husband and the only reason they do not do that is because the paternity of their kids would be ambiguous. Men not thus inhibited is the reason for polygamy. The original plan of the creator is monogamy and that is why gender ratio at birth is more or less 1:1 If God intended polygamy he would have made the gender ratio to be much tilted against men but presently we have countries like China even states in Nigeria like Kano where there is a shortage of women according to the census.