First, arousal does not mean consent Otherwise Dr. Kinsey's pedophiles had "consent" from infants.
Second, female-on-male rape does not require male arousal. She could be sticking things up his anus.
Sorry for the filthy details.
and the nerve endings still connect to the brain - even in someone with severe limitations - so some manual stimulation could easily result in the physical reaction she sought, even if the victim didn’t understand what was happening.