A rather crude mis-analogy. In that situation, a person who is drinking can take steps prior to becoming impaired so resolve the situation, i.e. they can take their keys and remove them from their person, or arrange another route home ahead of time. 

In the case of sexual assault, it takes two to tango. It is reasonable for the law to state that someone who is incapacitated from alcohol cannot give consent for sexual activity to another person, and that other person has the responsibility to obtain knowing consent. 

What happens if the other person cannot tell if the 'victim' is incapacitated? What happens if both parties are intoxicated? Good questions, requiring the application of judgment and context in the legal system. But it doesn't violate a general premise that those who are intoxicated are incapable of giving their consent to sexual activity with another and are victims, but nonetheless must be held responsible for driving while impaired.