Evolutionary biologists report that a human penis evolved, so that it can perform a variety of other important biological functions besides the obvious ejaculatory function needed to procreate. And they aren’t talking about pleasure.
Gordon G. Gallup and his associate researchers published several papers  on how a human penis evolved to help with the semen displacement strategy, which is important when more than one male has sex with a single female. While we aren’t going to dive into a discussion on whether monogamy is a wishful thinking amongst humans, ancient and modern ones, the reports say the following:
- Ancient women had more than one sexual partner. And a statistically significant number of modern women have more than one sexual partner as well (often in secret).
- The human penis head (glans) has evolved to displace from inside the vagina the semen of all the previous males who ejaculated there shortly before the present sexual partner. When the penis is thrusted in — it pushes the “old” semen to the sides, and when it is pulled out — the coronal ridge situated under the penis head scoops that previously deposited semen out. When the last man, a woman to have sex with, ejaculates, he delivers the load of his seed into an emptied vagina.
- Moreover as soon as he ejaculates, the pleasure disappears and he loses an interest in continuing the thrusting, plus his penis deflates, which ensures that he doesn’t scoop out his own ejaculate.
As a result of  and , the last male sexual partner is more likely to win the sperm competition and thus impregnate the woman with his seed.