You can't do a complete analysis of women's sexual choices without talking about class. Social class, that is; economic class.
Imagine what it would be like if we lived in a world where there weren't an effective way of preventing (heterosexual) intercourse from resulting in pregnancy. Imagine also that children's survival and success depended largely on their parents' long-term investment of both time and resources. Then imagine that there were dramatic differences from one male to the next in terms of the amount of resources they could offer. How would you expect women to behave?
Naturally you would expect most women to try to choose the best mate possible and to avoid mating indiscriminately. Still, women's behavior would logically vary by class. A woman who could expect to get a well-off man as a committed mate would have a strong incentive to please him by making it very, very clear that she's not sexually available to other men. Women whose prospects fall on the low end of the economic spectrum would have less incentive to make fidelity a priority -- in fact they (and by extension their offspring) might actually benefit if they opportunistically go from mate to mate.
Since privileged women are the ones with the greatest chance of attracting (economically) successful mates, we would expect to see sexual unavailability as the mark of being "high class" (for women) and open promiscuity as being the mark of being "low class."
Notice that (in this scenario) higher class women benefit from trashing sexually available women. Higher class women (and their offspring) are better off if their mates aren't directing their time and resources towards side dalliances. So privileged women have an incentive to promote the idea that promiscuous women are trashy and not worth one's time.
Men also benefit from publicly trashing sexy women (even if their private behavior is different). A high class man can please (or attract) a high class mate by making it clear he's not tempted by cheap sex. They also help convince their female relatives (sisters, daughters) to maintain a high class (less sexual) appearance to improve their chances of keeping prosperity in the family.
Is this the world we live in? Not exactly, but it's close.
For the reasons outlined above, humans have a natural prejudice towards seeing openly sexual women as inferior and less-valuable. Humans also have a tendency to come up with justifications for their innate prejudices (see my second discrimination against homosexuals article). Traditionally, the human prejudice against female sexuality has been expressed in terms of God and sin. Unfortunately, the hatred privileged women feel towards openly sexual women has also been dignified with secular feminist terminology, calling sexual women "objectified" (see Questioning Objectification) and claiming that promiscuous women lack "self-respect" and "self-worth." This last one is cruelly backwards as a "feminist theory" since feminists should be the last ones who would tell girls that their worth is based on their chastity and virginity (and corresponding marital prospects). (Here's a good example to illustrate how the twin doctrines of "objectification" and "sin" go hand-in-hand to condemn women and female sexuality.)
Now let's look at the modern situation.
Modern contraceptives have largely divorced sex from reproduction, and as a result the romantic/sexual aspect of our culture has changed in a fascinating way. For the long-term, most people (male and female) naturally want to be in a long-term committed relationship -- this is closely related to the human trait of feeling "in love." Humans are remarkably adaptable, and contraceptive use has caused males' romantic/sexual responses in our modern culture to evolve to some degree, but not as much as one might expect. Here's what I mean: A typical man is viscerally horrified at the thought of his mate being with another man sexually -- even though these days it's less likely to lead to raising another male's offspring. On the other hand, the preference for virginal, inexperienced females (as long-term mates) has proven far more malleable, to the point where many modern men actually prefer to marry a sexually experienced woman (see virginity: once an asset, now a liability).
At the same time, women's economic advances mean that women have greater power to set the terms and expectations for their own behavior and for their mates' behavior (see my post on gay marriage), or even to choose not to take on a long-term mate at all. So we finally have the opportunity to free ourselves from the ancient and medieval notion that a woman is "degraded" (reduced in value or class) by displaying her sexuality and engaging in casual sex. Parts of the feminist movement have embraced this positive development even though classist ideas (such as labeling heterosexual sex as "objectification" and "sin") unfortunately still persist in some feminist circles.
The irony of the feminist dogma of "objectification" is not just that it bashes women and female sexuality. Also, in reality women benefit when it becomes commonplace to display the female body in a sexual way. Allow me to explain:
In humans -- as with other great apes -- male sexual arousal is usually triggered by visual cues. The other great apes, however, can't control whether they're giving off visual sexual cues or not: a female chimpanzee has a red swelling if and only if she's in estrus, and there's nothing she can do about it. Since humans are at once highly social and largely monogamous, we've evolved a remarkable trait: human females can choose to display sexual cues or hide them at will (through the use of clothing, among other things).
Modesty is relative to culture (see topless on the beach). There exist cultures in which young women are generally prohibited from going out in public and where those few women who do appear in public must be covered in baggy clothing from head to toe. Traditionalists often believe that this is a good way to decrease men's inclination to view women as sex objects. This is an error. The real effect is exactly the opposite. Males are going to feel a certain amount of sexual desire no matter what. Taking the most obvious displays of female sexuality out of the public square merely changes the arousal triggers. It creates a situation where the mere presence of a woman -- even completely covered -- is sexually arousing. It takes away women's power to control whether they're giving off sexual cues or not.
Our society is a much happier place. Males, from a young age, are exposed to arousing images of women. This conditions them to require a fairly active display of sexuality (make-up and revealing clothing) before feeling aroused by the sight of a woman. I know many people will disagree and cite as an example some creepy guy they saw leering at every woman who passes. But as an obsessive people-watcher who has spent many years living downtown in more than one country, I can tell you that that guy is the exception. The overwhelming majority of men just interact with too many women on a daily basis to be aroused by every one of them. Let's face reality: a guy who fantisizes about having sex with every single woman he encounters wouldn't be able to function in society. It's because of this that the creepy, leering guy is the exception -- condemned by all -- instead of representing the expected male behavoir.
As a woman in a male-dominated field, I have a tremendous appreciation for how our modern society allows women to be seen as sexual on their own terms. On my own time I can put on a mini-dress or a bikini and enjoy the reactions I get. At work, I can dress in a professional way and expect that my male colleagues will treat me the same as one of the guys. This is because our culture allows for a huge range in terms of how much sexuality a woman can display.
So when I see a sexy woman in a bikini or less -- in real life or in the press -- do I want to degrade her by calling her stupid, exploited, and "objectified"? Not on your life!!! As a feminist, I have great respect for women who choose to display their sexuality, just as I respect the women who prefer rein it in a bit. That's what feminism is about: increasing women's choices.