Costaki was referring to the shape of a human penis. It may be an old theory, but a recent book entitled "Sex at Dawn: The Prehistoric Origins of Modern Sexuality" talks about it. The book talks about how in primates there are 2 types of male-female relationships. One is where the size of the male (being twice the size of females and bigger than all other males) ensures no other males can compete for females (i.e. gorillas). The other is where males and females are roughly the same size (i.e. humans), so other means for the males to ensure paternity are used. Gibbons are monogamous. Some apes' semen forms a plug to block other sperm. Humans evolved a uniquely shaped penis:
"Chapter 17 - The unusual flared glans of the human penis forming the coronal ridge, combined with the repeated thrusting action characteristic of human intercourse—ranging anywhere from ten to five hundred thrusts per romantic interlude—creates a vacuum in the female’s reproductive tract. This vacuum pulls any previously deposited semen away from the ovum, thus aiding the sperm about to be sent into action. But wouldn’t this vacuum action also draw away a man’s own sperm? No, because upon ejaculation, the head of the penis shrinks in size before any loss of tumescence (stiffness) in the shaft, thus neutralizing the suction that might have pulled his own boys back. Very clever."
Sadly, I'm sure 10 thrusts is more than many guys can last, but humans are still the love machines of the ape world:
"It bears repeating that the human penis is the longest and thickest of any primate’s—in both absolute and relative terms. And despite all the bad press they get, men last far longer in the saddle than bonobos (fifteen seconds), chimps (seven seconds), or gorillas (sixty seconds), clocking in between four and seven minutes, on average."