That the human penis might have evolved as a semen displacement device is "not an outrageously ridiculous idea," says Todd Shackelford, an evolutionary psychologist at Florida Atlantic University. Size, too, may matter here—the longer a man's penis is, the more likely it is to deposit semen out of reach of other men.
Of course, if a man has sex with a woman twice in quick succession, then there is the risk that he will displace his own semen. But researchers believe that men have evolved ways of preventing this from happening—for instance, men usually need to "recover" for a few hours between orgasms, and they rarely continue to thrust after they ejaculate. Both of these behaviors help keep men from accidentally removing their own goods.
Men also act differently when they have reason to believe that their partner may have cheated. Several survey-based studies suggest that the more time that has passed since a man has last seen his spouse, the more he will want to have sex with her. When men sleep with their partners in these circumstances, they also thrust deeper and harder than usual—consistent with the idea that they are attempting to "scoop out" rival semen—and release more sperm when they ejaculate. Not that men are aware that they're doing this.
If men have evolved adaptations to thwart women from having other men's children, what is to stop women from evolving counter-adaptations? Nothing. "You have what biologists call an evolutionary arms race of sorts," Gallup explains. Just as a man is more likely to initiate sex with his partner if he suspects she has been unfaithful, a woman who has cheated does the opposite, according to other research. She tries to wait as long as possible before having sex again, perhaps to maximize the chances that her egg will be fertilized by the superior male she dallied with. In addition, women are most likely to cheat when they are at the most fertile point in their menstrual cycle, and they're more likely to orgasm, too; some biologists argue that the female orgasm, which is accompanied by vaginal and intrauterine contracts, helps to pull semen into the reproductive tract.
Of course, there are other more obvious explanations for the waiting and the orgasms: guilt in the first instance and pleasure in the second. Perhaps, too, women wait because they don't want their husbands to smell the scent of another man. Cheating for evolution's sake is one thing; getting caught on an individual basis is another. A husband isn't likely to be more sympathetic when his wife tells him that evolution made her do it.