I did not actually masturbate with them.
I have a twin brother and back when we were kids we would always argue and carry on. Now we get along quite well. To this day he does not know about what I am going to tell you about.
At 17 Nathan had a buddy over one summer night for a sleep over. They were spending the night in the basement watching TV and were sleeping in bags on the floor. Being the sneak I was back then I decided to go outside and go to the window by the stairs to spy on them. I thought if things were quiet I would bang on the window and scare them.
I sneaked out the front door and made my way in the dark to the stair window. I got there only to find the guys were watching nude girls on the TV. I decided to watch now seeing how they would react to the video ignoring my temptation to tell my parents and stir things up.
As I sat there I could not hear anything because they had the volume so low. I could only see the TV screen and the two guys on the couch. Every now and then there lips were going and I wanted so bad to hear what they were saying.
After a while and just before I was about to leave I saw my brother stand up and take his shirt off. He stood talking and pointing at the TV then he undid his pants dropping them to the floor followed by his underwear. He leaned back with his arms wide open swinging his hips from side to side with a very hard cock sticking straight out and flopping from side to side! He then stepped out of his pants and sat back down on the couch. He said a few things back and forth then as he looked back at the girls on TV he started to jack his cock right there in front of his friend. I could not believe he would do such a thing but I also found it very interesting because I had never seen anything like that before. I saw picutes of naked guys but none in real life up to then. A few moments later Nathan's buddy leaned back and started to undo his pants too. He kicked them off with his underwear at the same time. He too had a long hard erection going on. He pulled his shirt off then they both started jacking off together. Now I was really getting turned on. I never knew back then that guys would do that together. I had never done it with any of my friends up to that time although I have a few times since and what a rush it is!
The two guys jerked their cocks for about 40 minutes and I watched every second of the show.
After a while Nathan picked up the coverter and moved the movie to a spot I'm sure they both liked a lot. There was a girl on her hands and knees getting it from two guys, one in the back and one in her mouth. They kept moving the movie back to the same spot then I saw Nathan's buddy lean back a bit while jacking a bit faster, then I saw his white cum shoot up a few centimetres into the air then cover his hand and body. I could see Nathan watching him too so he must have been interested just a bit in seeing him cum.
Nathan was not done though and he kept on jacking longer. It took him about five more minutes then he actually stood up while he squirted his cum. I was a bit shocked that he squirted out about one metre long two or three squirts and he let it go on the floor but he cleaned up soon after he was done showing off his squirting for his buddy.
That was it. I stayed in case more good stuff was to follow but they both put their pants on and turned off the sex movie to something else.
I went up to my room that night and masturbated myself like never before thinking about what I had just seen. That night changed the way I looked a guys and it also gave me the thoughts of masturbating with my friends when we had sleep overs. That is another story though.