I would think that the person's choice not to have sex would be more important than their choice not to become intoxicated.
If a person is intoxicated against their will, their "decision" to have sex with the person who intoxicates them is certainly not a valid one. If they choose to intoxicate themselves, they have decided for themselves to lower their resistance and inhibitions, so the decisions they make are much more valid.
For chrissake, if you (a theoretical non-drug user) got dosed with acid and went and killed a bunch of people, the person who dosed you would be more likely to get convicted than you would be.
Anyway, girls who get themselves really fucked up and fuck guys and regret it I have little sympathy for. Girls who get drugged against their will and fuck guys they wouldn't otherwise have been raped (if the guy she fucks was aware of the drugging, that is.)