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.
Different animals adopted different strategies to accomplish the same. For example dogs get locked to each other after having sex, thanks to the male dog’s penis expanding, so that the female dog can’t detach from it, thus ensuring that the semen gets into the cervix before another male dog gets his chance. And then you must have heard cats making strange noises when they do their sex play. This is because the male cat has penile spines on his penis, which roughs up the vagina of the female cat during the sexual act, thus probably ensuring that no other male cat will have a chance with that female cat for some time for obvious painful reasons. A feline research  suggests that one of the functions of the penile spines in the male cats is to trigger ovulation in the female right after the ejaculation. According to a recent DNA study  many eons ago human males had penile spines as well!
The studies  also discuss premature ejaculation. From one angle premature ejaculation seems to be beneficial from evolutionary biology point of view, since if a man took too long to ejaculate the tiger could eat him before he had a chance to do so. Yet, from the semen displacement strategy angle, being too quick doesn’t allow for enough thrusts to remove the previously deposited sperm by other males.
The studies also suggest that circumcised men are usually better at removing vaginal fluids and semen than uncircumcised men.
And another very curious observation is cited in the research papers – an uncircumcised man may impregnate a woman with a semen of another man, she never had sex with. Here is a possible scenario. Man A has ejaculatory sex with woman A. Shortly after that event an uncircumcised man B has sex with woman A. Chances are high that when man B disengages from woman A he will have some semen from man A lodged under his foreskin. Now if man B has sex with woman B, he could inject the sperm of man A into her. This scenario is much less likely to happen to a circumcised man as the sperm from another man is more likely to perish as it’ll be exposed to elements between the two intercourses.
It’s interesting to correlate these discoveries with the world of mushrooms, where a mushroom cap usually stores the spores. And of course some mushrooms look so similar to a male penis, that there is a whole genus of mushrooms, called Phallus (Latin for penis).
The illustration used in this article is of the mushroom called Phallus Impudicus.
 Gordon G. Gallup, et al – The human penis as a semen displacement device doi: 10.1016/S1090-5138(03)00016-3
Gordon G. Gallup, Jr., Rebecca L. Burch – Semen Displacement as a Sperm Competition Strategy in Humans. Evolutionary Psychology Vol 2, Issue 1 doi:10.1177/147470490400200105
Gordon G. Gallup, et al – Semen displacement as a sperm competition strategy doi: 10.1007/s12110-006-1008-9
 Aronson, L. R.; Cooper, M. L. (1967). “Penile Spines of the Domestic Cat: Their Endocrine-behavior Relations” (PDF). Anat. Rec. 157 (1): 71–78. PMID 6030760. doi:10.1002/ar.1091570111.
 McLean CY, Reno PL, Pollen AA, et al. Human-specific loss of regulatory DNA and the evolution of human-specific traits. Nature. 2011;471(7337):216-219. doi:10.1038/nature09774.