Evolutionary Psychology of Vigorous Sex

Semen Displacement Theory

Some evolutionary theorists believe that the penis evolved to remove the sperm any competing male may have left behind. And they’ve sought to verify the hypothesis by questionnaires:

These questions were meant to determine whether penile behavior (my term, not theirs) could be predicted based on the men’s suspicion of infidelity in their partners. In the first of these anonymous questionnaires, both men and women reported that, in the wake of allegations of female cheating, men thrust deeper and faster. Results from a second questionnaire revealed that, upon first being sexually reunited after time apart, couples engaged in more vigorous sex—namely, compared to baseline sexual activity where couples see other more regularly, vaginal intercourse following periods of separation involve deeper and quicker thrusting. Hopefully you’re thinking as an evolutionary psychologist at this point and can infer what these survey data mean: by using their penises proficiently as a semen displacement device, men are subconsciously (in some cases consciously) combating the possibility that their partners have had sex with another man in their absence.

Scientific American - a more than snickerful article: Secrets of the Phallus: Why Is the Penis Shaped Like That?