It's evolutionary, which doesn't mean it's good. Before birth control, if a woman cheated, her man might end up raising another man's child. Women face different risks, like abandonment while pregnant or with kids. This is why male and female jealousy is so different. https://twitter.com/R2daOH/status/963115716361601025 …
Kids complicate relationships a lot, no matter what kind you have. But I think 'hierarchical poly' is better suited to stable long-term child-rearing than 'relationship anarchy' is.