Trickle down effect.
Since all polygamous societies have increased genetic problems, the problem is not just marriage to cousins.
The genetic problems are caused by fewer males having all the children. The genetic problems compound down the generations as genetic problems spread throughout the population
All polygamy rots the gene pool.
Is this fair to children?
Nuclear family social structure is said to cleanse the gene pool in a few generations. The IQ soars and the genetic problems tank.