A society which permits polygamy results in a few high-status men having multiple women, and the average man having none. This leads to sexual frustration and other social pathologies.
Polygamy leads to a whole lot of unattached men of military age, with no prospects of happiness in this life. Also known as infantry, or more precisely, cannon fodder.
To reinforce your thought, polygamy allows women to be married in a society where low status men lack the ability and where with all to support a wife and associated family. Polygamy allows the basic inequality to be mitigated for the good of society