The notion of human sperm competition provides another reason why promiscuity makes sense from a reproductive standpoint. Defined as the “competition between sperm of two or more males for the fertilisation of an ova”, sperm competition results from the choice of a woman to have sex with several men. The “sperm war” she sets in motion helps her ensure that the most successful sperm cell will prevail and that she will conceive a baby with the contestant with the strongest army (made of millions of eager spermatozoa) fighting on his behalf. Since human seed can stay alive within the woman’s reproductive system for up to a week, the race can easily take several days. So if she has unprotected sex with more than one man within that time frame, she sets the stage for a battle of their gametes. The victor (there can be only one) gets to fertilise her egg (the Prize), thus initiating her maternity (the Quickening). Given that the winner has to beat not only the spermatozoa of its own kind but also those of one or more external rivals, the genetic material it carries must be of supreme quality. In line with the slogan “competition is good for business”, a promiscuous woman does nothing else than creating more choice for her valuable ovum, thereby improving the chances to produce more viable offspring. In this context, notice that the concept of “sperm competition” is different from the “sperm retention” theory, which states that women have more “high sperm retention” orgasms with their affair partners than with their primary mate (as measured by the amount of semen churned out immediately after intercourse). In addition, it appears that unfaithful women tend to time their sexual escapades with their lovers (that is, their orgasms) around ovulation, i.e., at that point of their menstrual cycle when they are the most fecund. Whether or not the decision is deliberate, these findings show that the wish for procreation can play an important role in adultery, and may also explain why ladies place particular importance on sexual gratification when electing their paramour.
As previously observed, women are essentially attracted to men who can provide two types of benefits: Plenty of resources and good genes. Alas, one single man is not always able to fulfil both requirements at the same time, which are often regarded as contradictory (think about the hard working nerd who neglects his personal appearance, or about the beefcake who prefers to spend his time in the gym rather than studying). Even if such a prospective mate does exist, he could be a philanderer or a “bad boy” himself, not willing to commit to the woman in question. Worse still, he might be so popular that he ignores her, or is not interested in her at all. In such a situation, she faces a classical Dads vs. Cads trade-off: Should she try to get an average-looking, but responsible provider husband who will offer her food, shelter, care, and invest in her progeny? Although he is likely to be a good father and a loyal partner, it is possible that he will score lower on the “health” or “gene quality” scale. Or is she better off with the handsome, masculine-looking hunk that will pass his first-class genes to her babies? The risk here is that he could channel some of his assets and sexual energy toward other females and their children. One way to deal with this dilemma is by opting for a so-called dual mating strategy and try to enjoy the best of both worlds. Under this scheme, a woman simply hooks up with several men: One who delivers the vital resources and with whom she is in an (officially) monogamous relationship; and a few other ones on the side who are in charge of impregnating her with healthy, fit and robust offspring, while at the same time delighting her with orgasmic pleasures. By spreading her luck between various mates, she not only garners genes from someone she considers as genetically superior but also makes sure that her successors will be safe, have enough to eat, as well as have a good education. Such a plan can only work out if her cuckold husband does not find out, in which case he could withhold his support and even banish her and her bastards. In other words, the dual strategy always comes with a number of risks. Nevertheless, if she is smart and cautious, she can probably get away with it.
Theoretical constructs such as the sexy son hypothesis also support the proposition that women are by nature interested in sexual variety. Due to the relatively long gestation period of human babies, it is not as easy for females to disseminate their genes as it is for males, at least not immediately. However, they can achieve this goal indirectly, through their sons. In order to increase the viability and reproductive success of their future generations, they need to select one or more casual sex partners who can help them bear sons who themselves will have a high value on the mating market. For that purpose, what else could be more effective than a casual fling with a well-favoured bad boy? According to the sexy son hypothesis, women who pick potential fathers for their genetic merits rather than their qualities as caregivers reap an evolutionary advantage. While this logic runs against the assumption that women nominate partners based on their ability to gather resources and on their willingness to make a long-term commitment, let us clarify once more that what is being said here only applies to the selection of a lover (not of a husband).
 Parker (1970)
 Baker (1996)
 Campbell (2002), p. 48
 Buss (2000), pp. 171-172
 See chapter 32 “Hearing something one hundred times is not as good as seeing it once”.
 Ridley (1993), p. 225
 Buss (2000), p. 173
 See chapter 22 “Man not bad, woman won’t bed”.
 Campbell (2002), p. 185
 Meston / Buss (2009), p. 14
 Titus / Fadal (2009), p. 64
 See chapters 14 “Fair lady is what gentleman seeks” and 19 “If you plant melons, you get melons; if you plant beans, you get beans”.
 Quirk (2006), p. 129
 See chapters 17 “Finding a good job is nothing compared to finding a good husband” and 21 “You can’t lead the life of a whore and expect a chastity monument”.