Wouldn't surprise me if that's about the same set of conditions that currently exists in large American cities, where a small percentage of men, both married and unmarried, have extra-marital sex with the majority of women, both married and unmarried.
Basically, you can outlaw polygyny, but the nature of mankind is such that it simply goes on anyway--though without a marriage license.
And it may also be, that in cases where the less successful men think they have found wives, that their wives cuckold them by getting pregnant in affairs with richer and more powerful men than their husbands, thus (in most cases, unconsciously) ensuring the resulting babies will have better genes.
Researchers have done genetic testing of neighborhoods and discovered that a substantial percentage (I believe around 25%) of children were not related to the husband of their mothers, though that would have been news to those husbands.
As for genetic testing in certain neighborhoods "proving" huge amounts of cuckoldry: I'd be skeptical of this so-called proof.
There's no up-side to these people and their families being genetically tested, no benefit for them: why would they consent to such a test? And it could only be done with consent.
And if they tested only consensually --- willing genetic sample donors --- how could the sample be either universal, or truly random? It would be a self-selected group, obvious volunteer-bias. This is methodologically bogus, the kind of fake research which is noised about by people promoting an agenda.