Women are socialized not to cheat. Historically women have payed a extremely high price for infidelity. Not so much men (generally speaking). The main reason for the difference in attitudes toward promiscuous/adulterous behavior among the sexes is the issue of paternity. In patriarchal societies a father's paternity is always in question, unless you can guarantee that your woman is only sexing you.
In matrilineal lines this obsession on paternity isn't an issues because succession is determined by the mother (which actually makes more sense) and there is no doubt that it is her child, hence the old saying, "Mama's baby, Daddy's maybe."
But...But...women need seek out a provider to help her care for her kids. True. But she also needs to seek out the best genes possible for her kids as well. And in many higher primate societies a females promiscuity ensures that her child lives:
In some primates, the promiscuous female mates obsessively with just about every male she comes across. She is, in other words, polyandrous.
This promiscuous behavior serves a vital function in primate social life — it ensures, for the female, that the paternity of her inevitable children can never be accurately determined, and all the males of the troop are equally likely to share food with her (and not kill her children).
And before anyone argues that that's just in primates...the number one cause of death for children 3 and under is being killed by men who aren't their father. It is pretty understood that (many) men don't deal well with other people's children, particularly male children.
And recently with the advent of the "at home DNA test" it's been discovered that as many as 10% of males are caring for children who aren't theirs, throwing that women are the more faithful sex out the window.
Women have a lot of social pressures to be monogamous and not promiscuous. Women get a lot of screwy messages regarding sex growing up. From good girls keep their legs closed ('till they find the one) to only bad girls perform certain sexual acts. Our desire for sex is often shrouded in shame. We regularly police each others sexual behavior, ready to unleash the "ho" label at the least sign of any transgression of established sexual mores.
On the other hand, our male counterparts are told to "sow their wild oats," and "not to settle down to fast." Men's sexuality is rarely shamed, unless it falls outside of sexual norms (i.e. homo/bi-sexuality) and in many cases is unashamedly encouraged. In this environment who is surprised that women are the more monogamous sex?
As many (but not all) of the traditional restrictions against female sexual behavior fall away, what you get is women who are as sexually promiscuous as their male counterparts. Women don't stand to lose it all (marriage, family, standing in the community) for their adulterous/promiscuous behavior. Often this "behavior" is labeled as "trying to be like a man" by those who believe women just aren't as sexual as men.
While either sex can choose to be faithful and non-promiscuous, both sexes are hardwired to do otherwise, strictly from a biological perspective. And the sooner that fact is accepted perhaps women can get away from the very restrictive sexual ideals many of us have been raised under and lead more honest, fulfilling sex lives that aren't shrouded in confusion and shame.