A large meta-analysis comparing these two measures found that they do correspond, but far from perfectly. On average, the correlation was stronger in men than in women, although even among men physiological response and subjective experience did not line up completely.
When they diverge, this is called arousal non-concordance — a mismatch between physiological response and how sexually aroused someone reports feeling.
It is especially important not to take the next faulty step and conclude that women “don’t know what they want” or that men always perceive arousal more accurately. The meta-analysis examined specific laboratory measurements and self-reports, and results can be influenced by methodology, stimulus type and other factors.
The practical conclusion is simpler and much more important.
An erection, lubrication or another physiological response tells us that the body responded. It does not speak on someone’s behalf and say: “I like this,” “Keep going,” or “I consent.” Fortunately, there is a more accurate source for that information — the person themselves.