I'm unsure about the existance of the condition (I come from a family of sleepwalkers/sleeptalkers, etc)
But then, regardless of it's existance, it cannot be used as an automatic presumption of consent. Consent should be active, willing (not just a case of "ohh, I'll just let him get this over with") and current.
For example, wrt current. You have the right to change your mind at any point. cOnsent cannot be presumed on the basis of "oooh, she was ok with it yesterday/eight hours/5 minutes/a nanosecond ago" If you're a sexomniac (trying to be courteous here, but it's a defense I don't believe in) then you are not in a position to gain consent. Therefore, you should be removing yourself from a position where you could rape someone.
If we put women iin a position where "sexomnia" is used as a defence for rape, under the basis that "he was asleep, he didn't know what he was doing", we demean every victim raped by their partners. It's the presumption of consent thing. You could see it now:
"She knew he had sexomnia, what did she expect?"
All the defense would need to do would substitute the first half of the sentence. Oh wait. That's what the rape apologists do already.
Little side note: Considering how clumsy sex can be, even at best of times, how do sexomniacs carry out their acts with such precision? I'm not sure I buy that...
Sorry, that's all jibberish, but still... I hope yoou get the gist of what I mean.