Evolution and natural selection are poorly understood. Many human behaviours seem disadvantageous when considered in Darwinian terms - but that's often because the type of environment that humans have evolved for, and the complexities of strategy are not understood.
Though evo psych explanations can be very silly, when they are reasoned carefully they can make a lot of sense. Principles of Game Theory should be considered, and in applying evo psych to modern behaviour, changes in environment and society are extremely relevant.
Just a quick note before I begin - natural does not mean good. Just because someone does something because of their genes, doesn't absolve them of any responsibility, any more than if they do something because of the influences of society. Another way of looking at it is that the wife throwing her adulterous husband's expensive shirts out the window is following her genes just as much as he was in cheating on her.
Evolution of Human Sexual Behaviour
What must be kept in mind is that humans have always lived in societies, for as long as animals that could be called humans have existed. Evolution has therefore acted on our behaviour in much the same way that social constraints have. Though all societies are different, they are all communities of individuals united to increase their chances of survival, through pooled resources, specialisation of labour, spreading of good and bad fortune, and safety in numbers, and thus share many similarities. The societies that humans evolved in were also all small, tribal, and hunter-gatherer economies.
The obvious difference between men and women sexually is that men's physical contribution to their progeny is a small teaspoonful of protein, wheras women have nutrients leached from their body for nine months of pregnancy and a few years of weaning. Beneath the wholesome maternal glow, the fetus is producing hormones to increase the amount of glucose in the bloodstream, where it can get at it, and the mother's body produces insulin to preserve her health for future children. The resulting evolutionary arms race leads to insulin levels during pregnancy of over a thousand times normal. And of course, if the parents split up before birth, we know which partner is left holding the baby.
What Women Want
Men's evolutionary success, then, is only limited by availability of mates, wheras women have a limited number of children to allocate. Studs stand to gain something that sluts do not. Women will therefore want to gain the most reliable and resourceful partners to look after their children, and the most desirable men to father their children and produce promiscous sons. These are often the same people, but a familiar pattern for female adultery is to seek a lover of higher status (and hence better genes) than her husband. The pattern can also be seen in differential preferences of women's casual flings, boyfriends and husbands.
What Men Want
Men on the other hand, seek to gain as many mates as possible, through any means neccesary. They also seek exclusive access to mates, hence cults and idolisation of female virginity, and the fact that polyandry is much rarer than polygyny. Monogamy was instituted amongst men like a cartel; if they all agree to limit their romantic endeavours, they have to work much less hard to secure a mate (in a polygamous society, the good men are never taken, and the losers can't even pick up a fat, ugly chick with low self-esteem). The temptation to cheat the system, as in any cartel, is always there, however, and many do. And when it comes to cheaters, men react with extreme prejudice. The modern phenomenom of large amounts of available, non-married sexual partners is probably unknown in pre-contraception times outside of harems, rape, prostitution, and libertine aristocrats.
"Adulterer" does have negative connotations, and I believe that "studs" do not have many pre-modern equivalents. The availability of single women of fertile age, willing to seek casual sexual encounters has changed greatly over the last few hundred years, particularly following cheap and reliable contraception, female emancipation, increased wealth and increased cost of raising children; that have caused people to not settle down with a life partner until much later. In modern society, any individual man who gets all the women does not have a large effect on your chance of getting a mate.
Studs tend to operate outside the bounds of marriage. They target single girls, not married ones. Sluts are ostracised by men as they are very likely to be already pregnant with another man's child; and they are unlikely to be faithful wives. Evolutionarily, being cuckolded is the worst outcome for a man. Men will not mind a one-night-stand with a slut, but will sniff at a longer term commitment. It's the infamous madonna-whore dichotomy; whores are attractive to a man, because, to put it crudely, they hope that history will repeat itself; wheras virgins make attractive spouses, as they are more likely to be faithful.
We men on the other hand have been too weak - we have failed to enforce collective punishments upon those men who sleep around.
Ask most men what they'd like to do to someone who slept with their wife. It would probably involve blowtorches and pipecutters.
My Explanation for the Double Standard
Married women are pressurised by their husbands and sisters to be faithful, though they will seek out a higher-status lover; and married men are pressurised by their wives and brothers to stay faithful, although they will be tempted to sleep with other women, or divorce their wife for a younger model; single women, although modern society may impress on them the joys of freedom, will desire to save themselves for the most eligible man they can get; and single men may attempt to sleep with as many women as possible, although if they cannot succeed in that game, they will advertise their worth as potential husbands and settle down with the best wife they can get.