People reproduced as best they could and marriage frequently had less to do with sex than it did with raw survival.
Should you have early Jewish-Americans in the line, you will immediately notice they married their cousins, and at early ages, there frequently being no other persons to marry. Or, if you have Mennonites, you will find the same thing.
Then, there are the American Indians - lucky to find a formal marriage there, even if there are records.
Do not judge the past, particularly the ancient past, by your own modern standards which arise out of a condition of wealth and leasure. In olden times farms needed worked, fields ploughed, cows milked, the elderly in the family needed attendants, and the only way you got those folks was to "grow your own".
It was done because Mohammed was a pervert for whom having a lot of other wives wasn't enough. I guess in his old age he needed a new "twist" to make up for the lack of Viagra.