Some evolutionary psychologists have proposed a specific adaptive function for the shape of the penis. The argument is that the glans, and more specifically the corona, scrapes and plunges competitors' sperm out of the vagina.
Scientific American did a piece on the theory, with commentary from the various researchers working in the area. Here are some of the highlights from the article:
It turns out that one of the most significant features of the human penis isn’t so much the glans per se, but rather the coronal ridge it forms underneath. The diameter of the glans where it meets the shaft is wider than the shaft itself. This results in the coronal ridge that runs around the circumference of the shaft—something Gallup, by using the logic of reverse-engineering, believed might be an important evolutionary clue to the origins of the strange sight of the human penis.
“A longer penis would not only have been an advantage for leaving semen in a less accessible part of the vagina, but by filling and expanding the vagina it also would aid and abet the displacement of semen left by other males as a means of maximizing the likelihood of paternity.”
The answer, according to Gallup, is their penises were sculpted in such a way that the organ would effectively displace the semen of competitors from their partner’s vagina, a well-synchronized effect facilitated by the “upsuck” of thrusting during intercourse. Specifically, the coronal ridge offers a special removal service by expunging foreign sperm. According to this analysis, the effect of thrusting would be to draw other men’s sperm away from the cervix and back around the glans, thus “scooping out” the semen deposited by a sexual rival.
They also suggest that it's possible for a woman to get pregnant from a man she's never had sex with:
Gallup and Burch also leave us with a very intriguing hypothetical question. “Is it possible (short of artificial insemination),” they ask, “for a woman to become pregnant by a man she never had sex with? We think the answer is ‘yes.’”
If “Josh” were to have sex with “Kate” who recently had sex with “Mike,” in the process of thrusting his penis back and forth in her vagina, some of Mike’s semen would be forced under Josh’s frenulum, collect behind his coronal ridge, and displaced from the area proximate to the cervix. After Josh ejaculates and substitutes his semen for that of the other male, as he withdraws from the vagina some of Mike’s semen will still be present on the shaft of his penis and behind his coronal ridge. As his erection subsides the glans will withdraw under the foreskin, raising the possibility that some of Mike’s semen could be captured underneath the foreskin and behind the coronal ridge in the process. Were Josh to then have sex with “Amy” several hours later, it is possible that some of the displaced semen from Mike would still be present under his foreskin and thus may be unwittingly transmitted to Amy who, in turn, could then be impregnated by Mike’s sperm.
The full article, including a description of the study using various dildos, an artificial vagina and starch goo, can be found here.