Scientists believe the shape of the penis may have evolved to help men remove the semen of love rivals during sex.
A marvel of evolutionary design?
Tests led a team of US researchers, headed by Professor Gordon Gallup, to conclude that the penis acts as a "semen displacement device" and its shape has evolved in part to displace another man's semen.
The team from the State University of New York believe the thrust of the penis during sex may help to clear a woman's reproductive system of a previous lover's semen.
They tested their theory in experiments using latex phalluses, an artificial vagina and a mixture of starch and water.
New Scientist magazine reports they found the coronal ridge of the penis, found where the glans, or head, meets the shaft, could scoop out more than 90% of the cornstarch mixture with just one thrust.
A phallus with no coronal ridge only managed to remove 35%.
They found the depth of thrusting was also important. A three-quarter thrust was found to clear out less than 40% of the viscous mixture.
They also say they have evidence, from surveys of students, to indicate that sex tends to be more energetic if the women is suspected of cheating, or if the couple has been apart - suggesting a sub-conscious desire on the part of the man to rid his partner of any trace of another's semen.
Lead researcher Professor Gordon Gallup told BBC News Online: "We theorize that as a consequence of competition for paternity, human males evolved uniquely configured penises that function to displace semen from the female vagina left by other males.
"Thus, the human penis may enable males to substitute their semen for the semen of their competitors.
"Not only did we obtain data derived from artificial genital models that were consistent with this hypothesis, but we also found that under conditions that raise the threat of female infidelity, college students reported changes in sexual behavior that would serve to purge the vagina of foreign semen prior to ejaculation."
'Far fetched' theory
Mr Derek Machin, a urologist at University Hospital, Aintree, said the theory seemed "far fetched".
"The research might very well be accurate, but I'm not convinced that just because the penis does something like this it was necessarily designed to have that effect."
Dr Colm O'Mahony, chairman of the UK Association for Genito-Urinary Medicine, said the theory seemed flawed.
"If the man continues to thrust after ejaculation he would simply be scooping out his own semen.
"Also does the sexual position matter - I imagine gravity has some role here. Maybe those missionaries knew something about position after all?"
The research is published in the journal Evolution and Human Behaviour.