(Original post by lightburns)
Some armchair evolutionary thoughts on the topic.
This all assumes that our ancestors were:
1. In need of two partners to raise young (likely, due to brain size)
2. Mostly monogamous, so low paternity uncertainty (likely, due to body-size sexual dimorphism and lack of penile spines)
Women might take the same strategy as blue tits. Blue tits have a faithful weedy mate, and then copulate with a strong mate who is not tied down. Weedy mate rears the strong mate's children, falsely thinking that they are his.
Men, however, have a much greater biological incentive for infidelity. If they can be the one to mate with another female, they can have their kids (and their genes) raised by another man! Whereas a woman only has an incentive to do this per child, and would have only reared 2-4 children to adulthood, there's only so many times that this is useful. Additionally, this is only useful at certain times in the month, as she is not always fertile (she becomes more attracted to masculine men whilst fertile).
The man, however, is always fertile, and will always benefit from extra-marital copulation.
Finally, the man has real reason to leave a woman who has been unfaithful. He would not want to rear someone else's children. Birds will leave a female who is shown to be unfaithful before she has her eggs, but the male will stay with the unfaithful female when he has the chicks, as he would lose his investment, and has no evidence that she was unfaithful at the time she had these eggs. Assuming this should also apply to humans, the female therefore has a biological incentive to curtail extra-marital copulations.
However, a female loses out if she leaves an unfaithful man, as the children would have required two partners, and there's no reason for a man to take on her kids. The man therefore does not have this instinct blocked as much.