this happened my senior year of high school and I haven't stopped jerking off to it three years later
My senior year of high school, my older bro asked me to watch his dogs one night while he and his wife went to a party. He's tall with broad shoulders, and is quite fit (he used to be quarterback at our high school). He said they'd be back around two, and to make myself at home while they were gone, and after they got back, I could crash on the couch.
So at about two they came home, his wife went straight to bed, so my bro thanked me for watching the dogs and went in the bedroom with her. I waited until 2:30 and assumed they were in bed when I decided to have a wank. I went to their computer (which is just outside the kitchen) and pulled up some porn pictures I regularly visited of some supposed brothers fucking. I got hard and started wanking really hard, thinking about my bro pushing his cock into my ass. I guess I got real into it and started to moan some, because all of a sudden I heard 'that you, dude?' I tried to cover my hard-on, but didn't have time to close the internet window. My brother came around the corner behind me and saw me scrambling to to hide my hard-on and the porn. He laughed and said 'I came to get a glass of water. What're you doing, man?' I told him that I had been jerking off and he apologized for surprising me. Then he smiled. 'You don't have to stop you know.'
My boner was aching inside my unzipped jeans, so I let it pop out. 'Nice' he said as he looked at my thick raging 6.5 incher. Then he looked at the computer screen. 'Do guys turn you on?' he asked. I blushed and nodded. He laughed again and set down his glass of water. 'I used to jerk off with some high school buddies of mine, in the lockeroom after practice or if we won a game.' he said. He smiled and glanced toward the hallway that led to his bedroom. 'It looks like I'm not gettin anything tonight. Mind if I join you?' My heart started pounding and I nodded. 'Here' he said as he bent over the computer and brought up another webpage. 'This is what I usually look at.' he said. I looked at the computer screen and it was pics of two dudes with HUGE cocks fucking the same chick. 'Just ignore the chick' he said and winked.
With that, he undid his pants and pulled out the biggest cock I've ever seen in real life, then or now. It must have been about 9 inches and uncut (I'm cut), and the thick shaft curved to the right. It was almost touching my shoulder and I was just staring at it, precum dripping out of my cockhead. 'You like what you see, little bro?' he said as he pulled his foreskin down the length. I nodded again as he said 'it's okay, dude, we're just guys. Your's isn't as big as mine, but it's still nice.' He smiled and winked again before looking back at the computer screen and pulling on his cock harder. Staring at my brother's huge cock it didn't take long for me to get close, so I pulled my shirt up and squirted onto my chest and stomach even though I still got some on my shirt. My brother watched me cum and then started breathing heavy. 'Hold on, little bro, I got something for ya' he said, and bent down as he shot a huge load all over me. It went in my hair, all over my clothes and cock and mixed with my cum on my belly. He laughed again and patted my back. 'Feel better?' he asked. I laughed and nodded again, not really sure what to say. 'Me too' he said 'Thanks, man. Be sure you clean up. Goodnight.' And walked back to his bedroom. After I heard the door close, I scooped up some of our mixed cum and started eating it. It was delicious, even better than just my cum. I used it as lube and came again in no time before passing out on the couch. I still have the t-shirt I was wearing I never washed it, and when I jerk off at school sometimes I put it on and think about my brother's huge cock shooting on me. We never talked about what happened and it hasn't happened again, but I wish it would.