In all species in which the female makes greater parental investment into the offspring than the male does (including humans and all mammals), is a female choice; it happens when the female wants it to happen and with whom she wants it to happen, not when the male wants it to happen or with whom he wants it to happen. The male has virtually no choice in the matter. A recent study neatly illustrates the principle of female choice.

In an article published in the September 2008 issue of Obstetrics & Gynecology, Bliss Kaneshiro of the University of Hawaii and colleagues study the effect of women’s body mass on their sexual behavior. Their sample contains 3,600 women of “normal” body weight (BMI < 25), 1,643 “overweight” women (25 < BMI < 30), and 1,447 “obese” women (BMI > 30) between the ages of 15 and 44. Their statistical analysis shows that there is no significant difference between normal-weight women, on the one hand, and overweight and obese women, on the other, on their , age at first intercourse, frequency of heterosexual intercourse, and the number of lifetime or current male sexual partners. It means that, contrary to what one might expect, overweight and obese women are not having later, less frequently or with fewer partners than normal-weight women. There is a significant difference, however, on whether they have ever had sexual intercourse with men. Overweight (92.5%) and obese (91.5%) women are significantly more likely ever to have had sexual intercourse with men than normal-weight women (87.4%).

Studies of mate preference throughout the world overwhelmingly show that men prefer to mate with women with low waist-to-hip ratios in the normal weight range. Men don’t like women who are underweight, and men certainly don’t like women who are overweight. So overweight and obese women could not possibly have as much sex as normal-weight women, let alone more sex, if men decide when and with whom to have sex. Most men would simply not choose overweight and obese women as their preferred sexual partners. Overweight and obese women can have more sex than normal-weight women only if women decide when and with whom to have sex, and men have little say in the matter.

When a man propositions a woman, she can respond in one of two ways; she can say “yes” or she can say “no.” When a woman propositions a man, he can also respond in one of two ways; he can say “yes” or he can say “yes, please.” He has no realistic choice to say no. Men may not be saying “yes, please” to overweight and obese women, but Kaneshiro et al.’s study clearly suggests that they are definitely saying “yes.”