Based on a real-life incident from the late 1980s, when I was in grad school.
I'd decided to stick around the University for the summer, since I really wanted to finish my dissertation and get the hell out of the decrepit East Coast city I'd called home for nearly five years. It was another stiflingly hot day, the fourth in a row over 90, so I decided to seek refuge in the air-conditioned library where I could concentrate.
I had my walkman on and was reading some boring book when I felt a boner coming on. That wasn't unusual; in those days I would beat off at least three times a day whether or not I had sex. I walked over to the men's room, but it was closed for cleaning. I decided to make my way across the plaza to the student union building, where I could also get a post-orgasm snack.
The heat outside hit me like a heavy wet towel. I moved as quickly as I could, and the sensation of the fabric of my shorts rubbing against the head of my cock made me want to get to my destination even quicker.
I opted for the quieter bathroom on the second floor. The student union building hadn't been remodeled like most of the other buildings on the campus, so air conditioning was pretty spotty, and was nonexistent in the bathrooms. I didn't care. It wouldn't take long for me to get off.
As I entered the bathroom, I was met with that unmistakable smell of piss, cigarette smoke, and a bleachy odor that didn't come from any cleaning products. Nobody else was there, so I chose the end stall, stripped off my shirt and my shorts, and sat on the toilet. I read the new graffiti on the walls, and admired the dried cum drips on the partitions. As was my habit, I scraped off some of the dried cum with my fingernail and licked it off my finger. Pity there wasn't anything fresh, or I would have licked that right off the wall. I'd done it before!
I heard the door to the bathroom open, and someone walked in. I stopped stroking myself, since I usually can't suppress a throaty moan when I came. I prayed that this guy would just be there to piss so I could get off and get the hell out of that stifling room.