Here at my university, we have posters up here and there indicating what "consent" means. If she's too drunk to talk, asleep, unconscious, or does anything except explicitly indicate "yes," then sex is actually rape.
Why? Because in a sexually charged, intimate situation, miscommunication is very easy. If expectations are mismatched, or if there's a slight difference in body language interpretation, miscommunication can cause heady fun to disorientingly transform into violation. Better to be a little awkward at first than to risk inadvertently hurting your partner.
Here's a simple story based on sexual dimorphism that predicts miscommunication. Men are bigger and stronger (on average) than women. A woman who is making out with a larger man she does not know extremely well is making risk assessments for the (low-probability, but very high-cost) case that the encounter becomes violent and balancing those against the (high-probability, high-benefit) case the encounter remains pleasant.
So let us suppose the woman does not give consent to sex. If the man continues to press for sex, the woman's assessment that probability that the encounter becomes violent rises.* One (common?) strategy is to continue to withhold verbal consent, but to grudgingly, unenthusiastically go along with intercourse, in order to avoid potential violence. This is being raped, and it's probably a much more common type of rape than stereotypical home invasions or abductions.
From the man's perspective, it looks exactly like he has simply convinced her to change her mind, except she's just less enthusiastic than she was before. He is making no threat assessments, so he judges her to not be making any either. She goes along with it; he would not go along with sex if he didn't want to have sex, so he judges that she is consenting. If she intended the "no" to be permanent, she would have reinforced "no" when he pressed -- that's what he would have done -- so he judges the lack of reinforcement as evidence that she has implicitly consented.
This is exactly what she intends! Causing him to believe she is consenting, even if she does not consent, removes the possibility that he violently assaults her. She is engaged in sex against her wishes, but has minimized harm.
Observe here that "male privilege" --- the man's perspective does not take into account the calculation the woman has to make concerning him --- leads to misinterpretation of behavior, and hence of the nature of consent. Whereas the behavior actually communicates, "I'll do it because I don't want you to assault me," the man evaluates it to mean, "I changed my mind to 'yes.'"
* You lose all the futures where he says "Okay!" and goes back to making out.