My husband has OCD that is often sexual, he fears being attracted to men and sometimes he will feel his body respond to seeing a man as if he was aroused. It is absolutely possible for your body to respond this way, even natural. If I were to sit here and worry/obsess about being attracted to women, all the while congering images of women in explicit positions to check myself, my body would most likely respond to such thoughts, but my mind would be repulsed.

People's body respond sexually to sexual thoughts/actions. Quite simple