Males and it's all because of testosterone.
Testosterone manipulation in eugonadal men has produced results consistent with the earlier hypogonadal studies. Bagatell et al. (1994a) used the gonadotrophin-releasing hormone (GnRH) antagonist, NalGlu, to suppress testosterone levels in eugonadal men over a 6-week period. This lowered sexual interest and associated sexual activity.
using weekly injections of either placebo or testosterone enanthate (200 mg) over 8 weeks, found no effect of the exogenous testosterone on sexual activity, either with a partner or as masturbation, but a significant increase in a measure of sexual interest independent of sexual interaction with the partner
Conclusions about the role of testosterone in the male The evidence is fairly clear that in men who have gone through normal puberty and who have not yet been affected by aging, testosterone plays an important role in their sexual interest and associated sexual arousability. The evidence points mainly to the effects of testosterone on central arousal mechanisms; the peripheral effects of testosterone in the human male, relevant to sexual arousal, are as yet unclear. It is also apparent that, in adult eugonadal men, the levels of testosterone in the circulation are substantially higher than required to maintain sexual arousability, suggesting that other effects of testosterone, most probably in the periphery, require higher levels than are needed in the central nervous system (CNS). The role of testosterone in the emerging sexual arousability of the peri-pubertal male is not well understood. In the older male, the picture is complicated by various aging effects, including altered hypothalamo–pituitary feedback, increased testosterone binding and reduced receptor sensitivity.
Increasing levels of testosterone occur in the development of girls as they approach and go through puberty. However, the changes are much less substantial than in the male. Testosterone starts at a lower level in the infant girl, and effectively doubles through pubertal maturation, compared with an 18-fold increase in testosterone for boys. The most substantial evidence of the relationship between testosterone and emerging sexual arousability in females comes again from Udry et al.(1986). As with their studies on adolescent boys, they found discrepant results between their cross-sectional study of eighth to tenth grade girls (approximately 13–15 years of age), where they found a relation between testosterone levels and measures of sexual interest and masturbation, but not with the likelihood of having experienced sexual intercourse, and their longitudinal study of girls post-menarche where the reverse relations to testosterone were found (Halpern et al. 1997). Similar explanations as discussed for their male studies could apply here, but in addition there is a crucial methodological issue of timing of blood sampling for testosterone in relation to the ovarian cycle (for a fuller discussion of these issues see Bancroft 2003).