IMPAIRED JUDGMENT. She was not fully able to make sound decisions. Some men take it even further and slip a "date rape" drug into the women's drinks.
Slipping in drugs without the victim's consent is one thing, but most date rapes don't happen like that - the most common date rape drug is alcohol knowingly consumed by the victim prior to the rape.
And now we get into slippery slope territory. If a woman is so drunk that she is slurring her words, vomiting, and has trouble walking, or even worse if she's asleep or totally incoherent, she's obviously too inebriated to give consent. But if a man and a woman have been drinking equal amounts of booze, and they're both at a BAC of ~0.12 (euphoric, low inhibitions, not able to drive, but still mostly functional - "tipsy" or "buzzed"), and they go on to have consensual intercourse while both slightly drunk, this cannot be construed as rape - even if one or both of them regretted it later.
But it's impossible to draw a firm line here, and the result is there are a whole lot of borderline cases. And there's a presumption of legal innocence, as well, which makes even the clearly across-the-line cases very difficult to prosecute, in the absence of witnesses, a positive drug test for a date-rape drug, or physical injury to the woman.
Here's a personal anecdote to underline the case. The most recent time I did anything sexual with anyone - which was over two years ago - she and I had both been drinking. We each had one of those 500 mL mini-boxes of wine - 2/3 of a bottle, or less than 4 standard drinks, which for me is enough to make me slightly buzzed, happy, and less inhibited, but still totally rational. She appeared to feel the same, and we ended up having sex. Everything seemed normal until afterward, as we were brushing our teeth and preparing to go to sleep. She was still coherent, but made a comment that indicated she did not remember something minor that had happened about 15 minutes before. I asked her a couple more questions, and I realized she was in alcoholic blackout. When she awoke the next day, she remembered the first half of the night but not the second. It was okay, because we had been in a short-term casualish relationship and this wasn't the first time, and she did remember things starting to get sexual. But she had no memory of most of what happened next.
She had had a
blackout, in which the short-term memory disconnects from the long-term memory. A person in an alcoholic blackout will usually seem normal (if a little drunk), because their short-term memory is okay and their long-term memory is too. It's just that short-term memories don't get recorded by the long-term memory. I had never heard of anyone blacking out at less than 4 standard drinks before, and I had gone drinking with her before, in larger amounts, without this ever happening. Apparently she had not really eaten dinner that night, and all the alcohol hit her bloodstream at once. She said afterwards not to worry and that she was really susceptible to blackouts. But blackouts have literally never happened to me, even during my stupid college years where I drank well past the point of vomiting and having the room start spinning.
This really scared me: what if this had been our first time together, and what if the blackout had started earlier? Would she think I raped her? This sort of thing happens sometimes, and it is absolutely a situation that could result in a rape accusation even though the man really would not be at fault - consent was granted from someone who was not visibly too intoxicated to grant it, but it failed to make it into her long term memory. And I don't even get around much - how often does this sort of thing happen to people who regularly go out and don't spend most of their weekends alone, making forum posts, reading books, and fooling around with chemicals?
My point is that it's a really gray area, and it's pretty much impossible for any third party to know the difference between my anecdote above and the sort of situation where someone is truly unable to speak coherently and is taken advantage of.