Marriage, by and large, exists to provide a stable legal and financial structure for raising children and families.

To that end, it's instructive to read Heinlein's novels The Moon is a Harsh Mistress, or Friday, to see discussions of polyamorous marriages and the various economic and social benefits they might provide in that regard.

Maintaining generational wealth is high on that list, as are the advantages of having multiple sources of income and having multiple caregivers.