Polygamy helped the israelite nation to grow, and later Jesus reinstituted the original sanctity of marriage. Still it's pretty wild to think that God allowed this but I suppose polygamy was likely extremely widespread and pervasive in ancient times prior to 1400BC that it couldn't be uprooted. The scriptures even says re: the hebrews "they are a stiff necked people" which I understand well. I can be really stubborn WHEN I'M RIGHT, but humble to recognize when I'm wrong.
Enough ranting.