Non-traditional marriages on Asgard

In my Norse mythology fanfic… I mean inspired story (with many liberties), the gods aren’t deities in a traditional sense but rather a more powerful than humanity race akin to elves, dwarves or aliens, with their own structured civilizations, politics and… how to put it… biological traits.

One of the latter is the gods’ drastically lower fertility compared to humans. For the worldbuilding reason, I went with it to avoid the issue of immortal beings possibly overpopulating the universe.

As for the in-universe stuff, the low fertility is still an issue that affects their society. During my story’s version of the Aesir-Vanir war both sides suffered great losses in population that would take a lot of time to return to previous numbers.

Then, when the Jotnar started becoming a more and more pressing issue, Odin, not wanting to risk bringing the Aesir and Vanir clans to a brink extinction too soon, created the einherjar and valkyries.

Another issue, that I am still uncertain however, is how to tackle marriage in such a civilization that is aware of its low fertility. Polygamy, open relationships and maybe even some leniency towards affairs feel like no-brainer, but I am afraid they don’t fit into a Norse myth-inspired setting, especially in a YA fantasy story that is not a dystopia.

Opinions?