In all cultures and societies, only women can be thought of as "sluts" or "whores", never men. Women are universally condemned even for the hint of promiscuity while it's almost accepted behavior for men (e.g., only women were stoned for adultery, not men). Many contend that culture is to blame for the double standard, but maybe only indirectly in a subtle way...
There's simply a fundamental difference in sexual libido between men and women in general. As one comedian put it, "when women think they're horny, they're like men are when they aren't even thinking about sex!". The obvious difference is that male lust is activated and fixated visually on the stereotyped female physique (large breasts, wide hips, long legs, buttocks and a pretty face); it's comparable to the "hunger" inspired by the sight of food. The mere sight of an attractive woman alone is enough to sexually arouse a man- erection and feeling of sexual lust- within seconds. Sexual intercourse is also extremely pleasurable for men because of the sensitized nerve endings of the penis; and most men can quickly achieve orgasm within a few minutes (followed by a period of refraction- loss of sexual appetite- and exhaustion, which is why men fall asleep after sex). To be blunt, sexual lust is a fundamental drive in men with priority only after thirst, hunger, sleep, health and physical safety. For women, though, sex is more like a "hobby". Now certainly, many are greatly enthused and even fanatical about their avocations (dancing, golf, jogging, surfing, skiing, tennis, stamp collecting, etc.); but there can be no real comparison between enthusiasm for a hobby, say, and genuine thirst for water.
Certainly, women appreciate "good-looking" men (and women); but women don't lust visually for the male physique. In men, after puberty, male hormones activate a part of the brain that is attuned to the female physique. Women have the same part of the brain too, but if activated, it'd be focused on female anatomy too. So if women were to lust for anyone, it'd be for other women (which would happen if they had sufficient male hormones). But female sex hormones don't produce an equivalent visual lust for the male physique; in other words, women simply don't lust for men or desire sex as men do. It's never been a case of women "teasing" men, i.e., withholding sex from men while secretly wanting it just as bad as men do except in escapist (male fantasy) pornography. So the "Hugh Hefner" philosophy that "nice girls desire and enjoy sex as much as men do" is simply untrue, but it's a convenient philosophy to have in order to justify philandering.
Women are sexually "aroused" by physical stimulation- foreplay- a long process (15-20 minutes) compared to a few seconds for a man. The sight of a naked man simply doesn't "do it" for women; compare the sheer overwhelming # of porn magazines of nude women vs. Playgirl (widely read by gay men); or the number of female prostitutes to male prostitutes, who mostly cater to a gay clientele. Also, in popular entertainment (escapist fantasy), women physically desire and enjoy intercourse as much as men do, but this isn't true. For one thing, the vagina doesn't have nerve endings comparable to the penis- at best, intercourse is painless with sufficient lubrication. Furthermore, nearly all women are incapable of achieving orgasm via intercourse alone- the clitoris is the female equivalent of the penis, not the vagina. Women require approximately 20 minutes of direct clitoral stimulation to achieve orgasm, so most women can only orgasm via masturbation or oral sex.
And yet, some men believe women enjoy "rough" vaginal intercourse; or are "begging for it"- i.e., want to be raped. This is just a delusion like paranoia or egomania. Many men also want to have a large penis (presumably so they give increased pleasure to their female sex partners), but I think men want it more for their own sake and self-esteem. Women can, of course, enjoy sex (just like we can all enjoy a swim or a walk in the park), but not with the frenzied desperation of men. I really think they mostly enjoy the close physical intimacy of hugging/foreplay; and, of course, they desire and enjoy orgasms too. But for women, orgasm is more of a "mechanical reflex"- a physical response to clitoral stimulation (it's more impersonal than for men). It's like throwing up by sticking your finger down your throat vs. seeing a repulsive sight. (Sex is far more visual for men.) Because of this difference in libido, men tend to view women as "sex objects"- not as people with feelings. They can't deal with an attractive woman without thinking of having sex with her in the background; I doubt many women have this problem with men!

In the debate on teen pregnancy, I've heard teens have sex because of "raging" hormones. This is undoubtedly true for most guys, who are so desperate to "get laid" they don't care about the consequences (pregnancy or disease). But for girls, who have sex- their motive isn't lust; but rather the desire for physical intimacy; peer pressure, simple curiosity, apathy and boredom; the belief they must "give in" to avoid being dumped by their boyfriends ("if you love me, you'll do it"); and to feel "grown up", "cool" and "popular". And some girls, esp. fatherless girls, are so desperate for male affection, they exchange sex for "love". Most girls seem unaware that a lot of guys will lie (and say almost anything like "I love you") for sex- i.e., "men give love for sex, and women give sex for love".

However, just because women don't lust for or enjoy sex as men do, that doesn't mean, of course, women can't be sexually promiscuous. After all, I'm not suggesting sex is an unbearable, disgusting ordeal for women like in the old Victorian notion; nor am I saying girls aren't interested in sex- it's just that statistically their interest is an order of magnitude less than men. Some women simply exchange sex for favors. There's the old Hollywood cliche that aspiring actresses have to trade sex for casting favors- just a business transaction. Most prostitutes are, of course, forced (even sold by their parents!) into it, esp. in 3rd world countries, because of sheer urban poverty or drug addiction (overpopulation- "excess supply"- cheapens human life I suppose). But some willingly choose prostitution, esp. the "higher-class" operations, because of the relatively high pay. For such women, casual sex, is just a job and "doing" a bunch of guys is no big deal to them. It's been said that men want sex for the sake of sex while women trade sex ("always in exchange for"). Prostitution and pornography then are simply the seedy side of capitalism (supply and demand). "Sex sells" and men are the main consumer target.

Sexual "modesty" in women then is primarily due to the fact women simply don't desire sex as men do. But it's also due to fear of pregnancy and disease; and because female virginity until marriage is promoted in much of the world by strict parents, especially by fathers. (Note: teen mothers tend to come from fatherless homes.) Many girls are raised to ...