Physiological reactions (erection or orgasm) result from physical contact, and sometimes even from extreme stress. When someone is attacked they can enter ‘hyperarousal’ mode, another term for the ‘fight of flight’ reaction. Physiological reactions can be a by-product of the ‘fight or flight’ mechanism our body uses to keep us alive, and does not reflect a victim’s actual emotional response to the attack or indicate consent in any way.
Our bodies react to stimuli; someone who is ticklish may laugh when they are tickled but that does not mean they enjoyed it. Physiological reactions to rape or sexual abuse work in the same way as this.