Bodies can react without permission
Genital blood flow, lubrication, erection, ejaculation, and orgasm involve automatic nerve pathways. Stimulation can activate them even when the mind does not want the situation. Fear and intense stress can also produce confusing physical reactions.
The same principle applies to tears, laughter, nausea, numbness, or inability to move. A response is something the body did, not a vote.
The rule that matters
Consent must come from a freely chosen agreement. It cannot be inferred from anatomy.