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Fidelity was, and was crucially important for women. The first requirement for getting men to raise children was that the men be satisfied that the children they were raising were indeed their own. Women had to establish their fidelity to the satisfaction of their mates. Men had more leeway because infidelity on their part did not tend to affect their families as directly.

You wouldn't even be asking these questions before ubiquitous birth control and paternity tests, the function of fidelity would be obvious.