Men have evolved different mate retention strategies to deter their partners from committing adultery, ranging from vigilance to violence. All of these are the result of male jealousy and paternal uncertainty (being unable to be certain he is the father of her children).
If a man’s partner is unfaithful and has a relationship with another man, he runs the risk of cuckoldry (that he may unwittingly invest resources in rearing children that are not his own). Male sexual jealousy may therefore have evolved to prevent infidelity by women and reduce the risk of cuckoldry