The concept that a woman's sexuality isn't her own to do with as she wishes:
A man who uses sex to get what he wants may be an asshole, but he doesn't lose anything of himself in the process. (Hello, James Bond)
A woman who uses sex as a means to an end whores herself out; it diminishes her value as a person, because as a woman her sexuality is apparently tied with her worth to herself and to others (especially herself, if she has any notions of decency! slutshameslutshame).
So bloody sickening.
Romance novels are usually chock full of female-sex-positivity and I love them for that, but sometimes the lingering negative aspects of the genre like the double standard mentioned above makes me want to tear my hair out and make a bonfire of every book that was particularly obvious in its transgression.
Palate cleanser time! Recs: Your Scandalous Ways by Loretta Chase deals with this double standard beautifully. Linked to my review, but it's more of a gushing declaration of love for the author than anything.