Oddly for a species of highly-altricial offspring, monogamy in humans is more social convention than it is innate. Infidelity is common, more often by men than women (though not by much).
Historically, men in dominant groups have been prone to coerce women with few resources and scant power into sexual relationships.
Despite laws against miscegenation – mating across racial lines – in the old South of early America, sex between male slave owners and enslaved females was widespread. One well-known example is “founding father” Thomas Jefferson’s repeated rape of his favorite slave, thereby producing 6 illicit offspring (along with 6 by his wife Martha). Jefferson started having sex with the enslaved Sally Hemmings when he was 44, and she sweet 16.
Humans are like many mammals in an ostensible monogamy honored in the breach. It is a considerable contrast to the true fidelity found in swans and several seabirds.
More significantly, despite the contribution to descendant fitness that fathers might make, their direct input into offspring well-being is often incidental, especially in comparison to mothers, who are far and away the primary caretaker. This too is typical of other mammals.