Let be begin with a disclaimer: I am a physicist, not a biological anthropologist, though I find the latter fascinating as well. As such, the argument I present here is intended purely as my own personal conjecture, although I also do not claim that it is my own original idea. I remember parts from an introductory undergrad class I took. The issue of women who cheat has been an issue in my family this past year and it has caused me to think a bit about the issue. This is my attempt to get it out of by brain. I welcome any comments or criticisms, especially by those with professional experience in biological anthropology.
The common perceptions are that men cheat and women don't, or that mean cheat because they are horny bastards and women cheat because they are looking to replace or find love that doesn't exist in their primary relationship. Both of these pop-culture, over-simplified ideas presume that men are generally horny bastards and should be ashamed of themselves for it and that women have no innate desire for sexuality on it's own. Rather, women usually do [or should] use their sexuality sparingly (since they don't have sex drives of their own), withholding it as much as possible to keep men interested or give them something to look forward to. This is the traditional (pre-birth control) role of sexuality in a romantic relationship. The opposite of this--women having sex sooner and more frequently in a relationship (or out of one), especially to indulge their own sexual desires--is frequently called "cheap" sex. My retort is this: to those use espouse the use of sexuality by females to entice men into relationship, nothing "cheapens" the idea and practice of sexuality more than using sex as a commodity to be traded for something else, whether it be relationship entrapment or cold hard cash. If you treat sexuality this way, the relationship you are trying to entrench is doomed from the start anyways. Rooted at the core of the practice is a fundamental disrespect for your partner. But this is not the anthropological part...
So what of the very common occurrence of married women who habitually cheat on their husbands, hide it well, and have no intention of leaving their relationships? The traditional view tries to pretend this doesn't happen, or if it does happen it is very rare and the individual women are mindlessly dismissed as "sluts"--a perpetually ill-defined term in its own right. But I leave that discussion for another day.
In nature, it is easy to understand why men cheat in their primary relationships. Survival of the species depends on competition to spread genes. Those who are best suited to survive and reproduce do so, beating out their immediate competitors and spreading "fitter" survival genes to the next generation. Human children need years of nurture to survive to adulthood, and thus an age when they can reproduce themselves. So the male-female long-term bonding is necessary for raising the children, but since a male can reproduce as often as he can copulate, having sex with many other females on the side only increases his genetic contribution to the next generation. Some of those other children may not survive to a reproductive age, but it's just a bonus on top of his contribution in his primary relationship. Women, however, need at least 9 months to have an offspring, and usually only one at a time. Thus, it is understood that women are far more selective in who they are willing to breed with; they only have a few shots at reproducing so they need to get it right the first time. This also assumes that women tolerate the cheating. I can think of a few reasons why, but that would be a digression from my current point.
So do the female humans in nature have a genetic, anthropological reason to cheat in their primary relationships? I conjecture "yes".
When I was an undergrad, I took a couple of those "Intro. to Anthropology 101" type classes and found them very interesting. I still remember one particular day when the professor claimed that there is one attribute in females that men are attracted to in every human culture on Earth, and that there is one attribute in males that females are attracted to in every culture on Earth. Men in every culture tend to be attracted to a female waist-to-hip circumference ratio of 0.7 (or something like that, if I got the number wrong). "Women in every culture are attracted to *he holds up his hand and rubs his thumb against his first two fingers, i.e. the "money" gesture*. So whether wealth is measured by money, yams (some traditional African cultures, thank you Chinua Achebe), or whatever else, women are attracted to that. This is also easy to understand, in a natural setting, the probability of an offspring surviving to reproductive age is far lower than modern times. If, then, females are attracted to wealthy males and find one to partner with, why would they cheat?
Remember what I said about females only having a few shots a reproduction in their lifetimes. The simple understanding is that these females find maximal probability of having offspring that survive to reproduction age by finding that wealthy male that can feed and protect them until they are old enough. However, I posit that there is a reproductive strategy from the females' point of view that offers and even better probability of having their offspring survive to reproduction. The man with the wealth and the man with the best genes are not always they same individual. The female's goal is best met when she copulates with the male seen to have the strongest genes (typically manifesting in what we now call sexual attractiveness), but has the wealthy life-partner raise the children thinking they are his. If this conjures the modern-parallel image of the wealthy socialite wife in a mansion nailing the buff pool-boy on the side, it should. Keep in mind also that this whole strategy is predicated on the assumption that the wealthy male does not know of her cheating, so women are also biologically selected and therefore predisposed of being very convincing liars concerning their faithfulness.
If you are a wealthy man and you have a wife that is considerably hotter than you are, you better damn well assume that she may be cheating on you eventually, if not already. You are deluded if you think otherwise, but the egos of many wealthy men--especially those born into wealth--leave them permanently deluded (and also cheaters themselves) anyways. It's natural.