Here’s what I just found. I think you may have misinterpreted Helen Fisher’s work:
“Mono” means one, and “gamy” means spouse. “It does not mean sexual fidelity,” says biological anthropologist Helen Fisher, a professor at Rutgers University in New York and author of several works on monogamy. Humans are among just 3% of mammals that form pair bonds, she says. Species like beetles, beavers and most birds share the instinct to partner up to rear their young.
It is a reproductive strategy, which Fisher posits evolved in humans because they walked upright. Unlike chimpanzees who carried their young on their backs, a human woman carried a child in her arms and thus likely desired a mate to protect and provide for her. Fisher notes that today, divorce commonly occurs in the fourth year of marriage, or about the length of human infancy. “Maybe a woman needed a man to raise a child through infancy,” she speculates.”