Thank you for asking. It shows a much greater level of concern for the welfare of the victims than those who just say, Can't happen.
What happens is: The perp take great pains to make sure that he is slow and careful about prepping his victim WITHOUT WAKING HER UP. If he is completely successful, he complete the entire act in utter secrecy, without even her knowing it has happened. This is what went on with Mary on the Dr. Phil Show, which her husband Steve fully confessed to on national TV. And I know it can happen, because my ex- used to do it to me.
What he would do is, first of all, absolutely INSIST that I sleep naked every night (regardless that I would wake up freezing and take forever to warm up and to get back to sleep again [after finally putting clothes on] in the middle of the night). On some nights, after I had said no for whatever reason, he would sneak it anyway. A few times I woke up to him prepping me with oral--told him to knock it off. But many times, because I was exhausted and in a deep sleep, he managed to slowly and carefully make his way through the whole process (I slept on my stomach most of the time). Then I would wake up to him putting a towel between my legs after the fact. I think it was the cold of his body having moved away from me that woke me then.
He even admitted to me in a frank discussion one time that the handful of times that I had been aware of him doing that was far less than the actual number. Also, he would at other times insist that, after sex, he stay joined inside me, spoon fashion, in order to go to sleep. Then sometimes, I would wake to him having another 'round' at it without my participation. To be honest, I don't know if my oldest son, in particular, was conceived in this way or not.
Mary, from the Dr. Phil Show, had been in absolute shock to find she was pregnant when, as far as she knew, they hadn't had sex for months. Then, as I said, her husband admitted on national TV having raped her in this way (though emphasis on the word rape was laid low, since Mary wasn't ready to absorb that ugly reality yet. The admission alone was shock enough).
Anyway, THAT'S how it happens. And if anyone had said such a thing to me before I had experienced it myself, I think I would have found it very hard to believe it also. Less for the supposed impossibility of the act, and more for the incredible creepiness of it! I mean, who on earth would think of such a thing??! But, apparently, some people do.