What does consent have to do with drunk driving? Lack of consent is an element of the crime of rape. Consent (or lack of it) has nothing to do with DUI.
I think the point is that if you get behind the wheel drunk, the law considers you to be still able to recognize whether you are capable of making this decision or not. Btw, it doesn't even matter how drunk you are, you are considered to be capable of understanding the consequences.
On the other hand, if one party gives consent to sex while being intoxicated, that consent is void if that party changes his/her mind later on...
In one instance being under the influence doesn't absolve you from your actions and in the other it does.