Double standard: Extramarital sex is worse by women than by men.
When a woman has extramarital sex, there is a chance that this will result in her husband raising a child that isn't his. But if a man has extramarital sex, there is no chance that this will result in his wife raising a child that isn't hers. This is a basic asymmetry. You could argue that contraception solves the problem. But our feelings evolved before contraception and the strength of our feelings are a result of the consequences in primitive times. This is why a cheating wife causes great emotional harm to her husband. Similarly, rape causes emotional distress to women because they lose control of choosing the father of their child. Rape with contraception doesn't reduce the emotional distress because this is a result of evolution, not logic.
What women really want from a husband is commitment. This is what they need to feel comfortable having children with the husband and raising a family. She wants commitment to be assured that the husband will always be there to support the family, especially when the wife is pregnant or with infants which put her in a weak position. So why are women so upset by male extramarital sex? Because modern women have been brainwashed to believe that male extramarital sex is an indication of lack of commitment. Historically you cannot find any instance of women complaining about casual extramarital sex by husbands before Paul started complaining about this in the New Testament. Of course women were jealous of mistresses throughout history, and this make sense since this is a real threat to her husband's commitment to her. It was Paul who introduced this terrible concept of male sexual fidelity to human culture. And even today, it is the degree of influence of Paul-based Christian culture on a society that determines the level of women's jealousy about male extramarital sex. Feminism is an extension of this culture and takes it to new extremes.