Hmm. Good question. Perhaps it is a reaction to having to take a piss in the morning? When I experience morning wood, I usually have to piss. Afterwards my morning wood disappears.
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Anonymous2010-01-01 8:03
But if the the bladder is full why do you have an erection? And it makes it harder to hit the bowl ;_;
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Anonymous2010-01-01 9:45
I believe it is a reaction to the surge of testosterone men get in the morning.