I never found peeing to be a particularly erotic activity, but one event altered my perception all at once. I was attending graduate school and at this school we had all of our classes during one day each week. It was a long day, going from 8am until 6pm and because of that, you got to know your classmates pretty well compared to other programs I have attended. But, back to the event.
This one day, classes just seem to drag and by the time lunch break came, I really had to pee. The problem was, everyone else had to pee too and there were long lines at the bathrooms. In the student lounge, there were 2 bathrooms...a woman's room with one toilet and a unisex bathroom which had both a toilet and a urinal. I had, along with lots of others, been waiting quite a while to use the bathroom and there were still 4 women in front of me so I was dancing a little. We all were trying our best to stay good natured about it and were picking on each other about having to go pretty badly. The woman's room opened first and one classmate entered it . Now 3 people were ahead of me but as the unisex bathroom opened, Marsha (the first lady in line) suggested that since there was both a urinal and a stall for the toilet, she and I could use that bathroom at the same time before any of us exploded. The other 2 women did not object to me cutting the line as they couldn't use the urinal anyway.
All four of us were saying that, after all, we were all adults and we just had to perform a natural body function.
It made sense to me, so I stepped up and held the door for Marsha, then held the stall door for her and after she locked it, I stepped to urinal and proceeded to unzip my shorts (it was summer), pulled out my penis and finally...relieved myself. All perfectly innocent...except as I finished peeing...I could hear her stream of urine hitting the water in the toilet and it was suddenly not so innocent for me. Intellectually, I knew it should not matter that this healthy, attractive woman was peeing only 3 or 4 feet from me, but I realized it did matter. I realized I hadn't put my cock away yet and looked down and realized I was in trouble as it had gotten rock hard and she would be done her business soon. I laughed nervously at my predicament.