My friend and I were jerk-off buddies for a while - about six months, not so regularly, but enough. One day we were camping and were with friends on a school trip and both needed to cum, being as horny as ever. He had a good plan - we would go to the shower block and do it together.
So we headed to the little building on the edge of our campsite, and I already had a raging boner. We were both about 6' tall but his cock was huge - around 7.5" already - and I was just 5.5. We took adjacent cubicles, only separated by one curtain for the two, and immediately I heard him jerking off. I peeked round the curtain and he was up against the wall with one hand and jerking with the other, pointing his huge cock at the wall. He looked at me and then my cock and I saw he had lubed himself up with shower gel, so I got mine and did the same in full view of him.
He turned to me and continued jerking and I slipped round the curtain and went to grab his and he grabbed mine first. I nearly came there and then. But I reached around his back and started jerking him off. He started moaning quietly and jerked me behind his back as he leaned against the wall again. I pushed my cock against his ass, up his back and between the cheeks, and started rubbing it up his ass. I came in about 90 seconds, in the end I was holding his hips and rubbing myself up him. It was the hottest thing I've ever done, coming on his back. He jerked himself off, squirting against the tiled wall.
We ended up doing much more than that, but it's the most intense thing I've done with a guy. I have a girlfriend now!