[TheMost]

The Visigoths smashed Rome while they were polygamous; once the Franks and Goths and Vandals were assimilated to the Roman system, and were forced into monogamy by the Church, then immediately after you start getting kings called "The Bastard". Meaning, even the kings were getting cuckolded, and doing nothing about it. Monogamy pushes the system too far. Every single stable monogamous society has a large contingent of prostitutes and unofficial mistresses. So the Beta males end up with the left overs in an even worse way than they do under polygamy; at least under polygamy you can get a virgin, or a widow who hasn't whored around. Having official wives is better for society; fewer fatherless men result. The Puritan lawyer "Martin Madan" documented this extensively in his book Thelyphthora.