It means that he's a rapist. Nobody can give consent to sex if they're not mentally competent. In your example, the man is drugging the woman in order to make her less resistant, or unable to resist, his advances.
Now, let's change this a bit. If a man deliberately gets another man drunk in order to have sex with him? If a man drugs another person, male or female, in order to have sex with him/her? If a woman gets someone else drunk in order to have sex with him/her?
How does changing the sex of the person who wants sex and the object of his/her desire affect your opinion of the act?