We Didn’t Evolve for Lifelong Monogamy

This headline is nonsense. It should tip you off that the author isn't a scientist. He's "Writer of fiction & non fiction".

My partner of several years is an evolutionary biologist. She says there's selective pressure for some monogamy and also some non-monogamy. Each has pros/cons and increases adaptability in certain situations. Evolved behaviors are never black and white or universal.

There's also not much evidence on what selection occurs after reproductive years end, but having more grandparents to raise children is likely selected for.