Completely true, only the names have been changed. Happened a few months ago.
My older brother and I have shared a room all our lives. His name is John. One night I was sleeping and heard a noise outside. It was John coming home. I got back to bed and covered myself with a several sheets, since it was winter. John came in but to my surprise he had his girlfriend Amy with him. What happened next was a stroke of luck and she sat on the bed. I could see his bed perfectly as they way I covered myself gave me an area to see through while my face wasn't visible from their angle. When I saw she took off her bra- which held pretty nice b-cups with pink, hard nipples. This insantly made me hard, so I decided I was going to pretend to be asleep and watch the show. My brother positioned himself next to her and took off his jeans. I could already begin to see his semi-hard erect penis through his boxers. Amy grabbed his boxers and pulled them down, and I saw his 6 inch uncircumcised cock, very similar to mine. She took hold of it and started jerking him off. By then my cock, (which I was somewhat elevated to find was a bit bigger than my brothers, was rock hard, I'm 6'5 inches and pretty thick and curved, his is 6'0, average thickness and straight, both uncut.
I grabbed my cock and started rubbing the head. I couldn't move enough to stroke it so I settled for massaging it. She continued stroking him, at times grabbing his balls with one hand. I got very into it, as the feeling of watching my very own brother made me feel guilty, but also extremely aroused. But I got a little too into it and began stroking, hard. I grunted a little, and accidentally moved the sheet as I neared cumming. In my excitement I had uncovered myself and alerted them to the fact that I was awake watching them. I lie there uncovered, with my hand in my boxers, with both of them staring at me with a 'what the hell?' look on their face. They were startled, and Amy quickly covered herself up with a pillow. I became red and starting apologizing feverishly. My brother put his boxers back on and told me to get out and that he'd talk to her.
A few minutes later he opened the door and we all began talking. I had thought that Amy was a typical 'good girl' so I was surprised at what she said next. She said that she understood that I was curious, and that she'd accept my apology, but only if she got to see my cock. That she wanted to know how I stacked up compared to my brother. I couldn't believe it, I was both terrified and aroused at the prospect. My brother told me not to worry and whispered something in her ear. She then took off her bra again and told me to touch her tits. I looked at John and again he said ok. I grabbed her perfectly round breasts with both hands, her hard nipples under my fingers. This made my cock as hard as it had ever been, as it shot up. She noticed and took off my boxers. She said it looked about the same, just a little thicker and began stroking it. I was in heaven and just went with it, paying no mind to John.
All of a sudden my brother took off his boxers again and started stroking his hardening cock too. He got near me and told me to jerk him off. I was hesitant, but Amy said she would stop if I didn't so I grabbed it. I felt strange grabbing his cock, feeling it throbbing, but I decided to just lay back and enjoy it. My cock was being stroked by Amy, in turn I was jerking of John, who began to finger Amy. It was ecstasy. I felt an explosion coming and mumbled it to Amy, she said not to worry. I shot my load all over her chest, with my brother following shortly after, cumming on her stomach. She cleaned up and they left afterwards, not saying much at all, only her telling me it was fun. We never did it again as they began living together shorty after, but it was an amazing night.
I consider myself straight, but after this experience, I think jerking off with a guy is as healthy an experience as could be. Experimenting never hurt anybody.