Mamzelle, you are absolutely correct about inbreeding. But there is much more to the demography.
Polygamy is a hideous social dynamic hardly ever discussed academically or religiously. Where it exists far too many men never have a wife or family, a recipe for violence/conquest. The wealthy men who do have wives acquire and dispose of women according to their fertile years. OBL had 53 brothers and sisters. This causes even more men to be useless, angry drones.
No father can properly raise several dozen children. Mothers must compete viciously for their child’s favorable outcome, so they inadvertently earn the distain accorded “trecherous” women.
In his Satanic wisdom, Mohammed designed an insatiable mercenary force with a sexual imperative to conquer, and if that is impossible they can have boys and female domesticated animals.
Criticizing polygamy should be acceptable due to genetics, anthropology and history. It’s been a colossal failure in other societies besides Muslim.
How common is it today?
Outside of Europe, polygamy was the rule, until modernizing natives decided to ape European customs wholesale. The problem with Islam has nothing to do with social mores and everything to do with Muhammad's statement to his followers that the most sacred form of Muslim religious worship is to make war on the infidel.