"To eat a woman out" is a metaphor. 

To Eat something is to take it within.  Ideas Are Food is a common metaphor, in which we talk about "chew on that for a while" or "digesting" something as we think it over.  If an idea is disagreeable, we can say it leaves a "bad taste in our mouths," another metaphor linked to this idea.

The idea of "out" is based on the image schema in-out, a very basic concept that we attain as babies and is the basis for quite a few of our metaphorical concepts and, hence, the way we think about many abstract concepts.  The most often used line in movies is "Let's get out of here!"  This is to say that "we" are within a container (a dangerous one in this instance) that we need to get "out" of.  To pull something out of containment is also to reveal and expose it.  Hence the phrase "come out of the closet."

So "to eat someone out" seems a metaphorical paradox, but I don't think it is.  Two constrasting metaphors can often function quite well in a sentence, because conceptual metaphors do not use all parts of the metaphor.  We can say someone came out of the closet, but we don't go on to say that within the closet, he wrapped himself on a hanger in darkness while creases came out of his flesh. 

So to eat a woman out means simultaneously to ingest her and to expose her.  "To devour a woman with your eyes" is a metaphor in which Seeing Is A Sexual Experience.  "To eat a woman" as a metaphor for oral sex is to experience a sexual experience.  To do this coupled with the metaphor of "out" is to expose her at the same time. 

Devour and expose.  Is the xray below a step in that direction?

Still under construction.