If you share it with the man your patriarch/ boss figure gives you to (Dany with Khal Drogo; Cat with Ned, etc., Margary to whomever daddy's decided to marry her to this week), don't reject them, and feel no disgust for them when they get a little rough with you ("she was glad he could not see tears in her eyes, etc."), and submit to him, then congrats, you are a good, smart, natural woman! Oh, but if you evilly try to manipulate men using sex, you will end up stupidly, evilly ruining your life, destroying all you love, losing all power, and walking down the street stark naked in a totally believable and not at all contrived scene in which the crowds mock your ancient 33 year old breasts.
Or else, using sex to get ahead is neither tacky, evil, nor wrong in a context where the society is totally patriarchal and women are treated as chattle. Because despite GRRM's didactic message, countless women did use sex or the promise of it to get ahead throughout history to raging success-- Elizabeth Woodville, Madam du pompador, Madam de mountspan, Anne Boleyn, and many, many others.
Many women who used sex to get ahead were not stupid or bad or "slutty" but merely canny and opportunistic. GRRM may portray Sansa as better, smarter, and more valuable because she does not use sex to get ahead and Cersei does, but I don't buy this. Sansa is undoubtedly better than Cersei because of her compassion, kindness, and intelligence. But I don't see why Cersei using sex to get ahead should even come into the equation.
The implication that Cersei is "old and used" (at 33? Seriously?) because she's slept with some men to get her way and she's over the age of 23 grosses me out.
GRRM obviously has a fear and hatred for women who use sex to get ahead, and these books portray Cersei as dooming herself (and deserving) the walk of shame for it.
In fact, one could argue that it was never her sexual deeds, merely the silly decision to rearm the faith, that did her in. If Cersei would have only kept the high septon down, she could have used that weapon between her legs to no particular censure whatsoever.
Of course, being so stupid and corrupt as to use sex to get ahead, perhaps the fact that she would be so brainless as to rearm the faith should go without saying. All that kettleblack shagging addling her already stone dull mind.