If the existing model is sexual dimorphism, with high sexual desire a male trait, you could simply suppose that it's a "leaky" dimorphism, in which the sex-linked traits nonetheless show up in the other sex with some frequency. In humans this should especially be possible with male traits which depend not on the Y chromosome, but rather on having one X chromosome rather than two. That means that there is only one copy, rather than two, of the relevant gene, which means trait variance can be greater - in a woman, an unusual allele on one X chromosome may be diluted by a normal allele on the other X, whereas a man with an unusual X allele has no such counterbalance. But it would still be easy enough for a woman to end up with an unusual allele on both her Xs.

Also, regardless of the specific genetic mechanism, human dimorphism is just not very extreme or absolute (compared to many other species), and forms intermediate between stereotypical male and female extremes are quite common.

I thought it was pretty clear. Sexual Dimorphism doesn't operate the way you think it does. Women with high sex drives aren't rare at all.

I have heard that, for most men and most women, the time of highest sex drive happens at very different times (much younger for men than women). This might account for the entire difference, especially if your'e getting most of your information from the culture at large. As TVTropes will tell you, Most Writers Are Male.