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.