An evolutionary psychologist would say that a man should fear his partner’s sexual infidelity because:

An evolutionary psychologist would say that a man should fear his partner’s sexual infidelity because:sexual infidelity could result in the use of his resources to raise another man’s child
Sexual infedility happens when someone choose to have sexual relationship with someone that is not their partner. 
According to evolutionary psychologist, this will be really disadvantageous for the men because they may have to make an effort to raise a child that is not his own