"FEMALE SEXUAL WITHHOLDING
Because sex is more highly valued by men and their access depends on the willingness of women to supply it, it can be seen as a valued good where women act as sellers and men as buyers, By withholding sex, women increase its value and render it a scarce resource, scarcity ratchets up the price men are willing to pay for it and this means pressure is put by women on other women not to undercut them, because if other women give sex away too cheaply, it reduces what women can ask from men. Monogamy is socially encouraged and enforced. A man is encouraged to stay with a woman because he is less likely to have opportunities to reproduce elsewhere. You will only get sex if you hang around and commit to that woman and, because all women are behaving like that, you are not going to have many opportunties for mating outside your mate.
Female sexual withholding is therefore key to paternal investment. Paternal investment facilitates reproductive success."