"Acquaintance Rape: Forced, manipulated or coerced sexual intercourse (or other sexual act) by a friend or an acquaintance. If the act occurs while the victim is unconscious, asleep or otherwise unable to communicate unwillingness, it is still considered acquaintance rape."
The fact is, if you do not see it being possible to "rape" your wife, then you are in essence saying that your wife has no rights to her own body.
Unless your girlfriend/spouse has specifically said to you "hey, if I pass out tonight, honey, and you still wanna get your rocks off, be my guest," while you're banging away at her unconscious body, you are technically raping her. She may not mind it when she wakes up, but you are also risking that she may be VERY pissed off, deeply hurt and literally feel violated that you would take advantage of her when she was in such a vulnerable condition.