I'm sick of hearing that men are "sperm spreaders" designed to have sex with every woman they meet. This is a gross distortion of what evolutionary biology says about human males.
Fish are "sperm spreaders." The females lay thousands and thousands of eggs. Males swim over them releasing as much sperm as they can. There is no connection between particular males and females. Fish parents do nothing for their babies.
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Human males need a different strategy. Women do not go into estrus. We are only fertile for a short period every month, but there is no obvious sign when this is. Couples who are trying to get pregnant often spend months having regular sex before they manage to make a baby.
When a human male has promiscuous sex with random women, he has no guarantee that he is impregnating anyone. Meanwhile, the random women are probably having sex with someone else.
A much better strategy, just from the view of reproduction, is to get a human female to stay with you and have sex repeatedly with you and only you. This gives you a pretty good chance to make a baby.
There is another huge difference between humans and most animal species. Our babies are born helpless. They need intensive care for the first few years and they need to be taught human ways for many years after that. As mammals, they also need a healthy mother who feeds them.
If a human male wants to be sure that his offspring will survive, he needs to stick around. He needs to make sure his babies are well cared for and educated - and that means he needs to be sure the mother is safe and well-fed. Even his grandchildren will benefit if he manages to keep their grandmother alive and healthy.
The most fit men are the ones who fall in love with their sex partners and want to do things for them. The longer they stay around, the better for their children and grandchildren. Pair-bonding is the best reproductive strategy for human males.
Cheating is a secondary strategy for humans. Even then, random promiscuity is not the best way for a man to produce offspring who survive. A better strategy is regular sex with a woman who is being supported by another man. Don't forget, though, the benefits of cheating always have to balanced against the potential costs, things like losing your own mate or being killed by the other man.
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It's not that humans are programmed to be perfectly faithful. It's just that the best strategy for us is pair-bonding and that is part of who we are. Sperm-spreading is for fish, not men.
Of course, whatever the best reproductive strategy is, humans are much more than our drives and instincts. We can think and learn and make moral judgements. We can be faithful because we love someone and don't want to hurt them or because we made a promise. That is part of our evolution, too.