(Peter Blockley August 2003)
Why do 50% of Australian females desire more than one sexual partner during their life? There appear to be two broad reasons why woman would benefit from copulating with more than one male. Females may trade sex for resources: money, food, a house, parental care or fertility. Alternatively, or in addition, females may engage in extra-pair computations in order to improve the genetic quality of their offspring. 
While this may not hold true in todays society, it provides good insight into how human sexual response has evolved. Females of almost all animal species routinely copulate with several different males . This polyandry leads to the idea of sperm competition, which is the competition between sperm from different males for the prize of the fertilization of the egg(s) produced by a single female. Male optimization of sperm concentration, copulatory plugs, the human piston penis and evidence of female polyandry support the idea of sperm competition in humans.
Optimal sperm concentration
Human males produce roughly 12.5 million sperm per hour , although only 1 sperm is required to fertilize the female egg. Studies on chickens show that the more sperm inseminated from a particular male, the greater his success in sperm competition . Studies in humans show that ejaculate quantities vary with the time spent with a partner. A man spending 100% of his time with a female will ejaculate 389 million sperm/ejaculate, while ejaculating 712 million sperm/ejaculate when only spending 5% of their time with a female .
The topping-up model predicts human males attempt to maintain an optimum-sized population of sperm in their partners tract as a defense against sperm competition. It was shown that over a fixed period (28 days) a male would inseminate roughly the same total population of sperm if they have four copulations during that interval or if they have eight copulations during that interval .
A method employed in some animals to reduce the risk of sperm competition is the copulatory plug. The animal sperm will harden in the female tract, producing a plug to ensure the sperm do not compete with other males. Human ejaculate coagulates to form a soft gel-like structure at the cervix. After 15-20 min the structure decoagulates and is ejected .
Dragonfly and damslflys prickly penises remove sperm deposited from a previous copulation . Similarly the piston shaped penis in humans would appear to serve a similar function. The human penis size and sharp are such that they provide a good fit to the female genitalia over ethnic group. This enables the penis to remove sperm deposits from a previous male through a thrusting action .
Studies of the blue tit (similar to a chicken) have shown that female blue tits initiated extra-pair copulation . This suggests females may gain from extra-pair copulation. Human females may initiate extra-pair copulation when they are unable to conceive. They may not be able to conceive because they are genetically incompatible with their male partner, as when the parents have different HLA haplotypes , or their partners are sterile.
One study followed the fate of 17 woman married to azoospermia men and seeking fertility treatment. No fewer than 4(23 per cent) of the woman became pregnant before receiving treatment and subsequently admitting to engaging in extra-marital sex. 
Both men and woman have nocturnal dreams leading to orgasm. More than 25% of devout Protestant woman over 30 have had nocturnal dreams leading to orgasm . Woman experience these dreams on average 2-4 times per year .
If females were monogamous, males would not go to extraordinary lengths to ensure their sperm fertilize the eggs. Sperm competition has evolved in response to both male and female polyandry. We must therefore question the notion of a monogamous human society.
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 Tim Birkhead, Promiscuity An Evolutionary History of Sperm Competition, Harvard University Press Cambridge, Massachusetts, 2000.
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 - Jequier, A.M., Non-therapy related pregnancies in the consorts of a group of men oserved with obstructive azoospermia; Andrologia(1985), 17, 6-8
 Alfred C. Kinsey, Wardell B. Pomeroy, Clyde E. Martin, Paul H. Gebhard, Sexual behaviour in the human female, W. B. Saunders Company Philadelphia 1953.
 R. Robin Baker and Mark A. Bellis, Human Sperm Competition: Copulation, masturbation and infidelity, Chapman and Hall 1995