This happened last summer. I was travelling throughout Europe by myself, and having a low budget, was staying at hostels during the night. It was also a fun way to meet people.
I spend the entire day walking from a side of the city to the other, and I was pretty tired when I got back to my room. When I arrived, some Australians guys were at the bar in the hostel. They seemed a bit younger then me, maybe 19, and were good looking (from a straight guy point of view). I joined them and we talked about everything, including girls. We talked about the nice ones we saw during the day and the fact that the European girls often go braless, to our great satisfaction I must admit.
We then headed to our room, and for a coincidence, they were in the same as I was. I took the bunk bed on the top, leaving one guy on the bed under me, and the other one on a separate bed next to ours.
Not being used to sleep in that uncomfortable bed, the guy under me seemed to find it difficult to find some sleep, and I heard some moves. Listening now much more carefully, I found out he was beginning to masturbate himself. I could hear the rhythm and the noise of his hand repeatedly touching his boxers. By that time, my cock was getting pretty hard and was requesting some fresh air too. So I slipped my penis out of my boxers and slowly began to move my foreskin up and down my shaft, trying to do as little noise as possible. I was beginning to breathe faster, and so was the guy under my bed.
I could hear him going faster, then resting for a while and, then going faster again. Listening to him made me so excited, my penis was hard and some precum was appearing, bringing some lubrication but also making some noise. I think he heard me masturbate, because a few moments later, he was making much more noise, as if he didn't care anymore.
He was getting faster, and my breathing was too. I was now masturbating with my entire hand around my dick, going up and down with the same rhythm as the other guy. And then, I heard him cum. That was such a satisfying moment, hearing the gasp he made and the noise his hard cock was doing while he was still masturbating, his hand and penis covered with some fresh cum, making a viscous and wet noise. Then I heard him cleaning the mess with some tissues he threw on the floor.
My penis was rock hard, and I needed to cum too... I was so horny that I decided to make some noise too. I was laying on the back, and was rubbing my penis with my entire hand, going up and down, my dick heading towards the ceiling. My precum was now getting even louder, and after a minute or two, I had to cum.
I continued rubbing my penis while my cum was rushing out of me. It was falling all over my black boxers and over my hand. It was awesome. I especially like the noise it made while I continued masturbating for a while after cumming, making sure the guy was hearing me.
I cleaned my hand on my boxers, leaving my cum on them, so he could see it when I would get up that morning. To my great pleasure, he indeed saw my cum on my underwear that morning, by showing me with his eyes where he left his...
That was the most exciting night of my trip... and I'll never forget that moment.