When I was 21, I had a girlfriend who had a brother three years younger than me. He was the only son and we get on very well. We went to the gym, to swim and talked about everything.
One summer, their family invited me to spend sometime with them at the beach. I shared the room with my 'brother in law'. As we had seen each naked before, we felt free to change clothes and be naked in our room. He told me once that he liked to be naked in his room and that at night he usually slept nude, so he said I could do the same. But I told him that, as I was a guest, I would prefer to wear something to cover myself.
At nights, we talked a lot and mostly about girls, sex and so on. I could see that he touched his dick during the conversation. Once, I noticed that his penis was getting hard and said something about it. He told me that he was quite hot but didn't cover it and asked me if I wasn't feeling the same. I was not so but seeing his dick getting harder and harder, I finally was.
His cock was almost full hard and I couldn't believe its size and told him so. He confessed his penis was 20 cm. long when hard. I was in shock. I had never seen a penis that long. I must say that mine is 16 cm. and I was always satisfied with that.
He asked me if mine was erect too and to show my dick to him. I took off my boxers and got up. He got up too and we started to compare our penises. Instinctively we both started to wank. We sat down in his bed and talked about which was our favorites technique to masturbate and even joked about the situation, until I told him that I was near to cumming and he said he was cumming too. We both finished on our stomachs and cleaned up with our underwear.
What I liked the most was that he told me that he enjoyed that very much. I was afraid that he could get embarrassed about it and said I liked it too.
We wanked together again in our bedroom and in the bathroom that summer and always after a long talk touching our dicks. One night I was exhausted and he masturbated alone.
Once at home, we returned to the gym and swimming and we arranged to watch a video and wank again. There were times that we stayed naked and didn't jack off and that made the things better.
After two and a half years, I broke with his sister but continued to see him. He told me once that I was like his older brother and that his was very grateful to me.
We are still very good friends and after 18 years we do not masturbate together but still see each other from time to time. We are both married and have two children each.
He is definitely my best friend and the person I know the best in this world.