I have experienced this phenomena many, many times and I'm sure it's not only me. I guess the sudden rise in arousal opens up the nostrils in a kind of fight or flight response, but I'm interested in the more intricate details of why this happens.
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Norepinephrine is responsible for, among many other things, causing a constriction the nasal vasculature and slowing capillary leakage that would lead to a runny nose.
Both the startle response (sympathetic) and the orgasm response (sharp transition between high peak of sympathetic response and descent into parasympathetic) result in large releases of epinephrine and norepinephrine into the bloodstream from the adrenal medulla.