 a gesture of trust and vulnerability. The woman lies on her back (usually) exposing herself without guard or guile to the frenzied thrusting of the man.
The man is also uniquely vulnerable at this time; at the point of ejaculation his back arches, his eyes close in rapture, and he is oblivious to all ordinary sensations. This vulnerability is exploited more cold-bloodedly in other species; the male spider and praying mantis pay the ultimate price for sex, for that gasping moment of desire, for the chance to perpetuate their genes; they form a tasty little post-coital snack for their partner.
And one of the consequences of the desire for sex is that sublimation of that desire has led to that finest of human emotions: romantic love, the passionate unconditional constant devotion to another person, the inspiration for great art, great literature, great poetry, great food, and great, great pop songs. Some authors, however, have argued that it is implausible to suggest that this remarkable harmony between the interests of the species and the ecstasy of the individual came about solely though evolutionary pressure; that the sheer joy of sexual love is far greater than can be explained by reference to biological utility or Freudian psychology — as Tina Turner rasped, ‘What's love got to do with it?’
As one of our great primal drives, cultures and religions have naturally developed many different ways to depict and control sex. For example, many traditional religions consider the act of sex without procreation to be sinful, whereas contemporary Western social mores and wider environmental concerns about over-population take the opposing view. In Western society, most forms of sex are now acceptable, so long as they involve consenting adults, and these increasingly bizarre forms of non-procreational sex may be a species response to overcrowding and an inherent, if unconscious, awareness of the dangers of over-population; they might be considered analogous to the legendary mass migrations and suicides undertaken by lemmings when their numbers become too great to support their food supply, though thankfully outre sex is much more diverting both to partake in and to watch.
So the bottom line, so to speak, is that sex represents a ferociously potent device for ensuring that our species continues to adapt and survive (and have great parties on the way).
In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety (known as a sex). Sexual reproduction involves combining specialized cells (gametes) to form offspring that inherit traits from both parents. Gametes can be identical in form and function (known as isogametes), but in many cases an asymmetry has evolved such that two sex-specific types of gametes (heterogametes) exist: male gametes are small, motile, and optimized to transport their genetic information over a distance, while female gametes are large, non-motile and contain the nutrients necessary for the early development of the young organism.
An organism's sex is defined by the gametes it produces: males produce male gametes (spermatozoa, or sperm) while females produce female gametes (ova, or egg cells); individual organisms which produce both male and female gametes are termed hermaphroditic. Frequently, physical differences are associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures the sexes experience.