Well, there isn't a shut-off valve to the cervix or uterus during arousal. Quite the opposite, really. And generally, with orgasm, flowe even often increaseas a little afterwards, because of uuterine contractions.
But menstrual flow is never completely contstant, not suring arousal or at other times: chances are you just haven't noticed that outside sexual situations because you're not really monitoring your secretions that intently at other times.
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