This happened last summer.
Every year my extended family gets together for a large family gathering at a camp a few hours away from where me and my family live. My mother comes from a large family, so I have many cousins, all around the same age as me (18).
I had woken up a little bit late, and everyone was already in the dining hall eating breakfast. I walked down to the bathroom to shower. There were two showers in the bathroom, situated along one wall beside the urinals. Each one was just a shower head and a drain in the floor, with a shower curtain for privacy. One of them had a sign on it reading 'out of order'. I quickly took of my clothes, put them on the cement floor, and hopped into the shower. I pulled the shower curtain closed and turned on the hot water. I stood under the water for a few minutes and was just thinking about having a wank, I hadn't had one in a few days because it's kind of difficult to do in a sleeping bag while sharing a tent with my younger brother beside me. My dick was just coming to attention when I heard someone call out from the other side of the curtain.
'Dude, you've been in there for like half an hour' the voice said 'I'm coming in'. Before I could do anything, Matthew, one of my cousins, was pushing the curtain aside and pulling off the pair of boxers he was wearing. He tossed the boxers out of the curtain and closed it behind him. Matthew was about my height, six feet, had the same brown hair, but was much more tanned and muscled, as he had been working at my uncle's farm all summer.
I stood there like an idiot, my dick sticking straight out in front of me, staring at my cousin, stark naked, just a few feet away. 'Shove over' he said. I did, and allowed him to stand beside me under the spray from the showerhead. I couldn't decide whether to stay or leave. If I left, I would miss the opportunity to watch my most attractive cousin shower, but if I stayed, he might think I was gay. I was spared the decision when he finally acknowledged my raging erection. 'Whoa, looks like you're a little excited there' he said. 'Yeah,' I replied, and probably blushed like an idiot. I was about to squeeze my way past him when he said 'it's ok, I need to bust a nut pretty badly too'.
I was speechless as Matthew leaned against the tile wall and began to rub his dick. I just stood there and stared. There wasn't a whole lot of room in the little cubicle, so I leaned against the wall across from him, but it was so small and we were both so tall that our legs were still touching. My dick felt like it was about to burst as I tugged on it, staring at the wet, naked body of Matthew beside me. We stood there for about five minutes, servicing ourselves, our wet bodies touching the cold tile and each other, wanking our dicks. Matthew started to grunt and his hand moved quicker over his shaft. His hips were bucking as cum erupted from his dick, landing on my leg and the tile floor. That pushed me over the edge. I experienced the most powerful orgasm I had ever had, and sprayed cum everywhere.
Matthew smiled, and said 'thanks', and with that wrapped his towel around his waist and walked out of the shower cubicle. I stood there for a few seconds and did the same. I had just stepped out of the curtain when Matthew left the bathroom, and was embarrassed to see one of my uncles standing at one of the sinks. It didn't take a rocket scientist to figure out he had seen Matthew leave the shower also. All he did was smile, shake his head, and leave the bathroom. I was left standing there, dripping wet and wrapped in a towel. I dried off, put my clothes on, and went out to breakfast.
I hope something like this happens again!