Why is ignorance as to whether your partner is too drunk to consent a defense to rape of an adult?

Jurisprudence 101
A person cannot lawfully consent to sexual intercourse if they are beneath the age of consent or too drunk.
Ignorance of the age of your partner is not a defense for rape.
Why is ignorance as to whether your partner is too drunk to consent a defense to rape of an adult?

References:
http://news.walla.co.il/?w=%2F10%2F2684018
http://www.jpost.com/National-News/Sudanese-man-charged-with-raping-17-year-old
State of Ohio R.C. 
http://codes.ohio.gov/orc/2907
--HT Rhys Cartwright-Jones.
http://www.avvo.com/legal-answers/if-you-have-a-drunk-girl-at-your-house-and-she-con-1368191.html