I've seen a friend's cock when I was about 15 (he was 16) and we enjoyed jerking together. It didn't last so much, after few times he stopped, as he was embarrassed. Later on we never spoke about that. Now I don't see him any longer, I know he's married with children.
At the school I've seen some mate's dick while changing for sport, because some cock popped out of boxer or brief without any intention. In the last years of the school some mate changed completely and openly and I saw their dick in the school locker room.
At the university time I roomed with a friend, he was quite modest and I was too, so he changed wrapped in a towel and looking at the wall ... I would have liked to see more.
Now with coworkers I time to time join the sport events and so far I had some good surprises, even if I still have to see some of them, who make me very curious.
I'm in couchsurfing network and it happened to me see some of my guests dick, as some of them are not modest at all and my couch is in the living room. I show off in the same way, if the situation is favorable.