It's something I've thought a lot about ThinkAboutit, in connection with a lot of the fanfic I read and the quite disturbing sexual politics of a lot of stuff written by women for women. I think part of it is the Christian purity thing: "if I'm forced (but not really because he's hot and I secretly want it) then I don't have to feel guilty." I've had some conversations with women, particularly in the Midwest, which shows the enormous extent to which this is still internalised. But the other frightening thing is the more I read and talk to people, the more I'm convinced it's a form of society-wide Stockholm Syndrome - if you can't make rape and sexual assault go away (1 in 4), some women handle it by eroticising it. It's a complicated, messy business psychologically, but for some women it's a way of taking back control in their fantasy lives of control that was wrested away from them in real life.
The other thing of course is the ubiquity of the male gaze in writing - a lot of women frame their sexuality in a way that is dictated by men. I quoted David Wong's parody at the beginning of the thread: "Janet walked her boobs across the city square. 'I can see them staring at my boobs,' she thought, boobily." But the sad thing is when you read fanfic, you find a quite substantial proportion of women writing for other women who will write this way. (The number of fics which involve a huge build up of sexual tension, then culminate in the heroine giving the hero a blow job, with no orgasm for the woman, is really extraordinary.)
Oh, and a side issue is of course simply bad writing. Our amateur author knows that A and B end up together, she knows that we, the readers, know that A and B end up together, so when she has (for e.g.) male B admiring then initiating intercourse for the first time with a sleeping A, that's all okay, because at some point in the future A and B are destined to be together. And the writer is too wrapped up in this future romance to have realised that at the time the scene is set, B does not know A's state of mind, so it's rape. And when you point this out to them, they get most offended, then start whinging about "but in the real world sex is messy and complicated and has grey areas." And yet another rape apologist is born, and one day they might end up sitting on a jury. 
It's all a bit depressing. Well, a lot depressing.
Disclaimer - there's also some bloody good fanfic out there, and a lot of stuff written by women who do care about consent and women's sexual agency, and writing three-dimensional female characters. And I am getting better at filtering out the crap early on and hitting the back button.