Evolutionary purpose of multiple marriages attainable via sperm and egg donation
This is a hypothesis. I may fledge it out later. Share any relevant literature. Discuss and treat it as your own.
There are examples of polygamy, polyandry, and group marriages throughout recorded history. It can be argued that these often served the purpose of allowing dominant male or female to create many offsprings with maximum genetic diversity.
Now in a more codified and urbanized society - a man or a woman who sees it fit to share her or his sperms or egg can do so through the medical system without requiring multiple marriages. Accompanying hypothesis: Burqas and similar attire were dresses desert-dwellers wore to prevent themselves from not-totally-unexpected sandstorms. Later as Islam spread, this continued for following two reasons - i) out of reverence to early practitioners of Islam, and ii) instigated on and supported with latter literature by certain males.