This happened on my eleventh birthday. I was starting to notice and get hard-on's quite often over the previous couple of months. Up until that day, I have touched and played with myself a few times but not to a real pleasurable level.
For my birthday, my dad took me and some of my class-mates (about twelve) to paintball. One of my friends (let's call him Jason) was wearing a beige colored pair of baggies and at one stage he bent down in front of me and I noticed the baggies were slightly see-through and I could see he wasn't wearing any underwear under it and could clearly see his butt crack through it.
I got turned on before when looking at nude pictures of women or when talking about girls and sex, but for the first time I actually got turned on by seeing another boys butt. I got an immediate hardon and for the rest of the afternoon I kept as close as possible to Jason to get more views of his butt. Before the day ended, I asked him if he wanted to sleep over and he wanted. He had never stayed over at my house before and I have hardly taken notice of him before at school.
We had to shower and change into clean clothes and with only five shower cubicles, we had to share with a friend and I chose Jason. We were told to shower with our clothes on to wash all the paint off it. When Jason's baggies got wet, they were totally see through and I could clearly see his dick, which was about three inches and tightly circumcised. Mine was only about two inches and also cut. When we changed into our dry clothes, he didn't put any underwear on as I expected so I asked him 'Don't you wear underwear?' and his answer was 'No, I don't have any.'
I had to quickly pull up my underwear and pants as I started getting a hardon again on that thought. The few times I tried to go without underwear before, my mom would shout at me when she noticed and she always seem to notice very quickly. When his mom came to fetch him, he asked if he could stay over and she agreed.
Back at home we played around, watched TV, etc. and when my dad sent us to bed, he told me to lend Jason some pyjamas, but before I could even get them out, he had already stripped and jumped into the spare bed naked. 'Don't you want pyjamas?' I asked and he answered; 'I always sleep nude'. I immediately got a raging hardon and decided to follow his example but turned away while stripping so he couldn't see my erection.
We started talking about girls and he asked if I wanked, at which I answered no. He said he has been wanking for at least two years and that I should try it. He threw his cover off, spat on his hand and started stroking himself. I watched in amazement as his dick grew about another inch by the time it was fully erect. I was squeezing my little erection under the covers and he said again I should do what he does. I struggled a bit but eventually got into a rhythm and after a few minutes got the most wonderful feeling in my crotch and had my first dry orgasm.
The following Saturday I slept over at his house and found that they were nudist, walking around nude inside their house nearly all the time. It took a bit of getting used to with constant erections the first few hours, which they acted as if they didn't notice. I had many sleepovers at his house after that with lot's of wanking and some more which I can't mention here, and never told my parents they were nudists because they would have stopped me going there immediately.