This is probably one reason I like to watch guys jerking off without them knowing.
I was about fourteen or fifteen when my uncle (my mother's youngest brother) came to stay with us. Since all the bedrooms in the house were taken he had to sleep in the den. My uncle slept on a couch that was opposite the wall with the door that led down to my basement bedroom.
One night I awoke and saw some light coming from under the door. I climbed a couple of steps until my eyes lined up with the crack under the door, then laid onto the carpeted stairs. I could see the couch where my uncle was bedded and being an avid reader, he had a paper back in hand. I almost returned to bed, but something caught my eye. I noticed that his other hand was moving swiftly up and down. I refocused and looked again. He was jerking off! The flood gates opened, the blood rushed and my dick hardened painfully quickly. I'd never thought of my uncle as a sexual being before, and man his eight inch hard dick looked nice.
As I lay there, my uncle continued reading and jerking, only stopping every few moments to turn the page. I was mesmerized, not looking away as I freed myself from my pajamas and underwear. As I kept watching, I wiggled my way out until they were around my ankles, at which point I was able to kick them out of the way. The pain in my dick from its sudden erection was replaced by the throbbing pleasure of an erection.
My uncle's rhythm was steady. A couple of times, he moved the book and stared at his dick as he pumped it. The book shook and his body rocked slightly in sync with his hand movements. The room was quiet enough that I could hear his hand on his cock, skin rubbing skin, with just a bit of pre-cum to make a bit of a squishy sound on each stroke. My own cock was very moist and slick in my right hand, while my left hand played with my balls and that sweet spot between them and my asshole.
I knew that he was getting close to orgasm because he put the book down and grabbed one of his white tube socks. He then closed his eyes and tilted his head and began to raise his hips to meet his hand on its downward stroke. His left hand moved down to play with his balls and cup them as he worked harder with his right hand. His fist traveled the length of his cock from the top to the bottom and back again. His breathing increased and there was slight moan, then he adjusted the sock. Within seconds a short but strong stream burst forth and landed neatly on the folded sock. Within a moment, two or three other spurts, each a little less than its predecessor, landed on the sock. Spent, my uncle milked his now softening cock for a few seconds, pulled up his briefs, and turned out the light.
I found my way back to my bed through the pitch black room, wishing that I'd cleaned up more that day. I prodded under the bed until I found that special use towel, and finished myself off with a couple of strokes. My cum missed the towel and landed on my forehead, my face, then my chest and finally the towel. After catching my breath and cleaning up I found my pajamas and returned to sleep.