When I was in high school, people made fun of me for being fat and awkward and unfashionable--I was never gonna get laid, haw haw.
When my little sister was in high school, people made fun of her for being pretty and outgoing and a fashion plate--she's such a slut, haw haw.
This is the Slutadox: if a woman isn't sexy and doesn't have sex, that's terrible. But if a woman is sexy and has sex, that's terrible. It's a Madonna/whore dichotomy where being a Madonna is no good either. If you wear a long skirt you're a prude and if you wear a short one you're a slut, and sometimes you find there just aren't any lengths in between.
I never understood the problem with being a slut, to be honest. It's very weird to get insulted with "you get laid a lot!" Um... thanks? But my secret theory is that this isn't the patriarchal possessiveness thing it appears to be. My secret theory is that men hate sluts because sluts are heartbreakers. You think you're really special and worthy for a girl to sleep with you, and then you find out that she sleeps with lots of people, and it diminishes your specialness. If sex is a meaningful thing for you, finding out that it was meaningless for your partner is painful--legitimately so, sometimes. But admitting that you wanted meaningful sex and that you're emotionally vulnerable is not manly, so instead guys just scream "SLUT!" like it's just intrinsically wrong for a woman to have an interesting sex life.
The temptation is to try and escape the Slutadox by appeasing it, by being extremely moderate. No makeup would make me ugly and too much makeup would make me slutty, so I'll perfect the art of subtle natural makeup. Not fucking dates until we have a relationship is prudish and fucking on the first date is whorish, so I'll fuck on the third date. But this appeasement always leaves you on a knife edge, always vulnerable to the people who think third-date sex is whorish, and completely shoves aside the question of how sexual you want to be. Not that there's one answer anyway. I've fucked guys the day I met them and I've fucked guys I'd been friends with for years, and I've had both go well because they were different people in different situations.
And the scariest thing about the Slutadox is that it crops up within feminism. Oh, I'm not one of those man-hating legs-not-shaving feminists! But I'm not one of those party-girl "sex-positive" fun-feminists either! Even within a movement of women's liberation, women are still expected to carefully calibrate their level of sexiness.
I don't know. Do I even have a sexiness level? I've had sex with approximately 26 people, I'm hyper-orgasmic and kinky and have a sex blog. And most days I don't wear any makeup and I shlump around the house watching "Mythbusters" while doing beadwork. I'm not sure I can stick a pin in one spot on a Madonna/whore continuum and go "yep, that's me."
At both ends, the Slutadox is really just about finding reasons to judge and hate people--well, women--for whatever they do. Don't participate in that crap. Whether a woman is sexy or unsexy or some of each or anywhere in between, going "god what a prude/slut" is pure cruelty and hypocrisy. And holding yourself to "mustn't be a prude, mustn't be a slut" standards--that's just self-hatred.
How sexual your behavior and self-expression are is your own business. It's all okay. End the Slutadox.