Bill Poole's blog: "human"

Human sperm is of a low quality - relative to many animals, including primates, a low proportion of the sperm is viable - and this encourages pair bonded sexual partnerships to the detriment of any arrangement of irregular and infrequent coitus, particularly harem-like arrangements and surreptitious affairs.

Women's visual and olfactory sexual preference is biased toward desirable partners during the period of fertility and toward less desirable ones the rest of the time. This is likely an adaptation running counter to monogamy, which tends to maintain the procreative availability of desirable partners and the pacification and utilitarian engagement (household protection and production) of undesirable ones. Underscoring this adaptation, incidence of orgasm in females is predominantly dictated by the perceived sexual (hereditary) desirability of the male, and household partnership or emotional involvement is not a predictor. This is significant because the vaginal and uterine contractions that accompany orgasm promote successful fertilization. Exerting an opposite influence on the probability of successful fertilization is ``flowback'', by which unwanted semen is expelled from the vagina.

Human ovulation is concealed, not advertised. This is unusual among extant species, and the development of this adaptation underscores how vital it is to the evolution of the species that women maintain substantial, practical, if largely unconscious control over their impregnation.

The temporal window during which fertilization is possible ends just 24 hours after ovulation. Only during this period, the uterine mucosa drop their immunity-enhancing viscosity sufficiently to allow penetration by sperm. Desired sperm that arrive prematurely are waylayed and suspended in pockets in the cervical wall, and are reanimated by a signalling mechanism when the fertilizable egg makes its entrance. The pistonlike thrusting of the penis during coitus, combined with the foreskin behind and under which debris is accumulated, constitutes a pump that works to remove seminal matter that might have been deposited by a sexual competitor. Additionally, the duration since the male last engaged in coitus with the instant partner modulates the volume of his ejaculation, to better the chance that a competitor's attempt at fertilization will be thwarted by his own.

Oral sex is a practical behavior. Through various sensory channels - but primarily, through olfaction and taste - the one playing the oral role is able to gain information revealing the health of a present or prospective partner, and often, whether he or she has recently engaged in sex with someone else.

Masturbation in males serves to purge the seminal tracts of sperm that is no longer viable, and is perhaps useful in purging the system of other perishable fluids. Male masturbation culminating in orgasm (and concomitant ejaculation) is only possible with direct physical manipulation. Masturbation in females serves no practical purpose, and orgasm can be attained completely in the absence of direct physical manipulation.