*Please note: this writeup is not intended to condone infidelity, nor is it designed to provide lame excuses for people who want to cheat and get away with it. If you are getting into a serious relationship and don't think you can be faithful, at least be honest about it. It saves all involved parties lots of heartache.
People in long term relationships cheat. Various researchers (Athanasiou et al, 1970; Hite, 1987; Hunt, 1974; Kinsey et al, 1948 and 1953) have estimated that between 41 and 75% of married men and 18-70% of married women will have at least one extramarital affair. There are thriving prostitution and pornography industries everywhere. Human males have (relatively) huge testicles designed to produce large amounts of sperm (Smith, 1984), release more sperm per ejaculation the longer they have been separated from their mates, and men seem to have one type of sperm that is suspiciously efficient at blocking sperm from subsequent inseminations (Baker and Bellis, 1995). When offered sex by a complete stranger, 75% of men said yes, and the declining 25% were typically apologetic, and not wholly disinterested (Clark and Hatfield, 1989). Women are more likely to have orgasms with lovers than husbands, and unconsciously time encounters with lovers to coincide with ovulation (Baker and Bellis, 1995).
If you have ever cheated or been cheated on, you are probably intimately familiar with the pain and anxiety experienced by the cheated, and the damage that infidelity does to a relationship. So why do people cheat at all?
Taking an evolutionary perspective*, the simple explanation is that people are more likely to have a successful offspring if they reproduce with more than one partner. The specific hypotheses about why cheating would make successful offspring more likely differs between men and women.
For males, an increase in the number of sexual partners corresponds with an increase in the likelihood that he will have intercourse with a woman during the brief period when fertilization is possible. He also has the opportunity to impregnate multiple women. If he succeeds, then not only are there more copies of his genome in the next generation, but maternal genetic anomalies inherited by one offspring will probably not be present in other offspring (since they had different mothers), therefore increasing the chance that at least one of his offspring will itself be reproductively successful. Indeed, men have historically been more inclined to seek more partners than to try to have more children with one partner (Betzig, 1986).
Females can also benefit from increased genetic representation in later generations as a result of reproduction with multiple partners. As with men, having offspring with multiple partners decreases the probability that all of a woman's children with have the same genetic faults, thereby increasing the chances of at least one successful child. Also, the paternity of children of women who have multiple partners may be difficult to determine, thereby increasing the chances that more than one man will invest some kind of resource (be that money, food, protection, etc.) in her offspring (Hrdy, 1981).
Additionally, both genders can benefit reproductively if they manage to have a sexy offspring as a result of an affair with someone really sexy.
So why does everyone cheat? That's obvious: because we are all selfish jerks.
*There are probably problems with the use of evolutionary theory to explain infidelity. If you know some good ones, add a writeup!
Athanasiou, R., Shaver, P, and Tavris, C. (1970). Sex. Psychology Today, June, 37-52.
Baker, R.R. and Bellis, M.A. (1995). Human sperm competition. London: Chapman and Hall.
Betzig, L.L. (1986). Despotism and differential reproduction: A Darwinian view of history. Hawthorne, NY: Aldine.
Buss, D.M. (1999). Evolutionary Psychology: The new science of the mind. Boston: Allyn and Bacon.
Clark, R.D. and Hatfield, E. (1989). Gender differences in receptivity to sexual offers. Journal of Psychology and Human Sexuality, 2, 39-55.
Hite, S. (1987). Women and love: A cultural revolution in progress. New York: Knopf.
Hrdy, S.B. (1981). The woman that never evolved. Cambridge, MA: Harvard University Press.
Hunt, M. (1974). Sexual behavior in the 70's. Chicago: Playboy Press.
Kinsey, A.C., Pomeroy, W.B., and Martin, C.E. (1948). Sexual behavior in the human male. Philidelphia: Saunders.
Kinsey, A.C., Pomeroy, W.B., and Martin, C.E. (1953). Sexual behavior in the human female. Philidelphia: Saunders.
Smith, R.L. (1984). Human sperm competition. In R.L. Smith (Ed.), Sperm competition and the evolution of mating systems (pp. 601-659). New York: Academic Press.