Mate guarding is behavior that men perform to protect their mate from sexual advances from other men and to prevent their mate from falling in love with more dominant men. The man not only mate guards as a means of paternity assurance, but also to save face, to avoid being outcompeted by other males, and to avoid having to start all over again courting a new partner (something women do not need to worry about with their passive sexuality).
Women also mate-guard, but to lesser extent. In one survey, 53% of women, but only 13% of men would forgive their partner's infidelity. This is likely related to the fact that humans are a moderately polygamous species. Women in a harem would be used to their man having sex with other women. There is also a crucial sex difference in that women can always be certain that their children are theirs. Men cannot get pregnant. Women seem to be genetically oblivious to this fact, e.g. women more likely to evade a partner's mate guarding efforts.