I agree.
I don't know Morrison's intent, but if I had to guess I would say that the detail is not intended to be salacious, but to give the reader nowhere to hide. It's so explicit and shines such a bright light on the act that there is nowhere to go, no way to read a euphemism and gloss over it in your mind. It's very disturbing and reading those passages made me feel like like I was crouched in a spotlight, trying to find a place to hide, wondering when it would end. Perhaps it intended to provide a much milder parallel of the victim's experience. Very effective and disturbing, if that was the intent.