It's called paruresis. Happens to me all the time. Even after I've started going, if someone pulls up next to me, the stream gets thin and slows. So there are muscles involuntarily tightening up inside. It's embarrassing as hell, because since the inability to pee may make me look like a perv (just standing there with my dick out), it shrinks to even smaller than when swimming in cold water. No fun. I blame it on my mother (of course, right? lol). When I was like seven, I went to pee after taking a shower and it stung. So I called mom into the bathroom to ask her wtf; she had no idea what could be wrong, so she sat on the edge of the tub and waited for me to pee again to see if it still hurt. Nothing came out - foreshadowing my adult life, for sure. Had she not been divorced, my dad would've said something to the effect of, "You just got a little soap in the hole. No problem." Oh the damage done by ignorant women.