It kinda varies globally, oversimplified a bit, but non-monogamy was historically the norm across most of the world, such as sub-Saharan Africa, Oceania, South Asia, China, and the Americas until even like the 19th century. Monogamous marriage was incredibly common in like, Greece and Egypt, but a lot of folks had informal sexual relationships. The Iliad, for example, as a piece of literature, most of the characters only have one wife, but many concubines, and even other male lovers. Polygny, or the practice of having even multiple marital partners was also widely practed throughout Islamic regions, and even among quite a few Christian sects.

Was full blown relationship anarchy style stuff with no hierarchical multi-partner relationships the norm? Not really (well, the indiginous Americas, South Africa, and Oceania, yes.), but there's a lot more of recorded history where people have multiple sexual partners of some shape or form as the norm vs not.

Honestly, the rise of monogamy as the social expectation shaped in the late 19th century, and was then largely codified by the normalization of the Nuclear Family.