the fact that men and women are on average equally promiscuous,

It depends what you mean by average. It is true that, if the sex ratio itself is 1:1, the mean number of heterosexual partners honestly claimed by men will be the same as the mean number of heterosexual partners honestly claimed by women. But the mean is not a very informative statistic in this context: it tells us nothing about inclinations to promiscuity. The median and the mode are more informative of interesting differences in sexual inclinations, and there is no reason to expect them to be the same between males and females. They are up for empirical measurement. To take an extreme, in a harem-based society, a minority of males has a larger number of sexual partners than the mean, the majority of males have fewer sexual partners than the mean. All the females in a (strictly policed) harem have only one sexual partner. In a society where a minority of females are prostitutes the balance shifts again, and so on.

Richard