This is a true story that I have never told anyone before.
When I was 19 and my little brother was 18, he would jerk off every chance he got (as boys at a certain age do) and there were multiple occasions that I'd walk in on him doing it. We shared a bedroom with bunkbeds for years which made privacy difficult, for both of us. At first, I thought it was disgusting and I'd get so mad at him for letting me see him doing that. He got better about it after I flipped out one time after yet again rounding a corner to the sight (keep in mind I was NOT trying to catch him in the act), and I didn't catch him again for a long time, even though he was obviously still doing it a lot. He'd gotten better about locking the door or waiting until everyone was asleep. My parents also eventually had an addition done to our house which gave me my own room (finally!).
Because of all that I never had to walk in on my little brother rubbing one out.. until a few years later, that is: I'd come home for the holidays from college and by this point he was getting ready to go to college himself. On the first night I was home I got up in the middle of the night to go pee. I hadn't seen him yet because he'd gotten home late after I'd gone to sleep. In fact, he didn't know I was home.
Anyway, as I was walking by his room I looked in to see him there in the dark sitting in front of his laptop wanking away. I assume he wasn't worried about getting caught by my parents because the master bedroom had it's own bathroom and they are heavy sleepers.
I continued by quietly and used the bathroom, then for some reason I can't explain, I didn't flush as I knew he would hear it. I walked very quietly back down the hall and listened by his open door, hearing the clear sounds of his hand moving up and down on himself and the computer mouse clicking. I slowly peeked in and just... watched. He was facing slightly away from me so I couldn't see his face, but even from this angle I could see the top half of his cock in the light from the monitor and I could tell it was MUCH bigger than anything I'd ever seen in person. I wasn't exactly aroused per say... just curious for whatever reason. So I kept watching. I'd never done ANYTHING like this before, mind you.
As I watched his hand go up and down, my eyes were locked on his penis and my heart was getting faster and faster in my chest. Suddenly he pushed his chair back a few feet and leaned back in it with his legs spread, and his wanking pace increased.
With a stifled groan, his cock erupted. And I do mean erupted.
More cum than I thought possible shot straight up into the air. Rope after rope of it. Some all over himself but lots hit the hardwood floor to, making loud splatting sounds. I couldn't look away until he finished, panting from the orgasm.
That's when I looked at his screen. He was looking at my facebook when he came! More specifically, a picture of me and my best friend at the beach in our bikinis!
I sneaked back to the bathroom and waited a minute then flushed. I immediately heard his door close and I quickly went by back to my room to bed.