Fidelity is most important for a woman for avoiding confusion- under normal circumstances. Confusion because, if she were to make love to more than one on the same day or thereabouts, the father of the issue may be a cause for doubt, thereby causing confusion. The least that a parent could do is to tell the child who the father is. Since it is the mother and only the mother who could certify that, it is essential that she be sure of the facts.
But if a man were to have sex with two or more women and the woman were to have it with one alone, there would be no such confusion.
Hence recommending fidelity in a woman has the effect of avoiding confusion. Thereby bringing stability to the society. But women demanding fidelity in men, has no effect other than social implications.
It is organic in a woman and inorganic in a man.