woman have plenty of sex drive, we just are often pickier than men! but trust me, woman are also always thinking of sex
I think I heard somewhere that its strongly linked to testosterone levels, and I that (as someone else eluded to earlier) that its also to do with the amount that is present in the body compared to the levels of other hormones.
This is why TS folks quite often have quite messed up sex drives whilst they are making the hormonal transition form one gender to the other...as far as I know.