> At lease in the case of traditional marriage, there's the creation of children to contend with.

Huh? Sterile people have always been permitted to marry. Always. Everywhere. Under all circumstances. Ever. Sometimes inability to bear children has been grounds for divorce or--in the case of royalty--annulment, but there has never anywhere been a test for non-sterility prior to marriage. So what does "the creation of children" have to do with "traditional marriage", given "traditional marriage" could and did trivially take place between sterile individuals.

And whose tradition? One man, one woman, the man's concubines, the odd houseslave, and perhaps a sheep or two? Is that the traditional marriage you're talking about? It's by far the most common kind over most of the world before industrial capitalism.

"Why just two people?" is of course trivially easily answered: humans are a socially pair-bonded species. This seems to be basic to our biology, and is the case in every society ever observed. While bonds between more individuals are possible, they are rare and complicated (the modern poly community is small, for example, and most frequently consists of multi-pair-bonded networks).

> Sometimes inability to bear children has been grounds for divorce or--in the case of royalty--annulment

Not just royalty. An annulment means that there was never a marriage…because a marriage which cannot produce children wasn't regarded as a marriage (or as less of one), which suffices to demonstrate a strong connexion between childbearing and marriage.

> "Why just two people?" is of course trivially easily answered: humans are a socially pair-bonded species.

We are also, you may have noticed, two-sexed. This is even more basic to our biology.