Physiological response and subjective arousal do not always match.
A person can have an erection or vaginal lubrication in a situation they absolutely do not want to continue. And the reverse can also happen: someone may feel strongly sexually aroused while their genital response is weaker than expected.
It is particularly dangerous to treat an automatic bodily response as evidence of consent: “But you were wet,” “But you had an erection.”
That tells us nothing about whether the person wanted what was happening to continue.
The body can respond automatically to a sexually relevant stimulus. Consent remains a person’s decision, not a reflex of their genitals.