Two situations from my history:
1. In a movie theater restroom along a wall of urinals without dividers. I was there already and alone. An older man (60+) came in. He chose one near mine (with only one urinal separating us). He hauled out a huge piece of meat and I was extremely hard pressed to not look at it. Of course he noticed; I couldn't stop staring. Finally, as I was doing the post-pee shake, I looked candidly at his thick willie, up at his face and said, "You win!" He just smiled and laughed.
2. At a place of employment I was in the restroom where there are only 2 urinals (non-divided) and 2 stalls. I was peeing and in comes a fellow employee, loaded up with several computer bags/cases slung over each shoulder. I suppose he thought it would be easier to stand and pull out his dick than have to unload the bags so he could fit into a stall, because he squeezed in next to me.
All was well until quite suddenly all the bags slipped down both of his arms. Both of us responded by reflex, dropping our dicks and reaching for bags. Naturally, both our dicks were bobbing and I was lucky enough to not have sprayed piss all over the place as I turned to help at the moment.
By the time we got all the bags back over his shoulders, we had each seen the other's dicks very, very clearly. (Yes, I was larger, but not by much. Perhaps I am just more of a shower.) Anyway, he was obviously shaken and apologized for the event and also, albeit awkwardly, for how it put us both on display. I said something like, "Oh, no problem. I'm probably the best guy for this to happen to, because I don't care who sees me and I don't mind seeing other guys." I think I weirded him out with that one. Needless to say, he always used the stalls after that.