Often, they are str8 (or say they are), but they like to show off in front of other guys, like 'accidentaly', or just 'joking', or more openly, especially in a locker room, or other guys-only places. So, have you met, do you know a guy like that... And as a rule, they are packing some stuff down there. Have you fantasized about him afterwards etc?
And as a rule, they are packing some stuff down there.
Oddly enough, the guys I knew that liked (or seemed to like) showing off were the guys with the least impressive packages. Although it was always after a cold swim practice, and obviously they were never erect, so maybe they just seemed small.
One guy in particular, I remember, had the biggest pube bush I've ever seen in my life. It was especially weird considering he was naturally smooth almost everywhere else on his body. But he had so much hair around his cock and balls, and it was always so poofed out, that the only thing you could sometimes see was the very tip of his cock pointing out.
He was one of the few that was comfortable being completely naked for a few seconds (or even a minute or so) while we all changed, so I had plenty of opportunities to really check him out. Another teammate of mine had the first uncircumcised dick I'd ever seen, but he was always really quick to put his boxers on after pulling off his speedo, so I only caught glimpses of it.
Most of the guys hid their dicks by putting their shirt on before taking off their speedo, then leaning slightly forward while putting their undies on (so that the shirt would hang over their crotch). This is what I usually did because I often had at least a semi going. I noticed one dude was always super-careful about changing this way, so I always did my best to catch a peek at what he was hiding. The few times I did, I could see that he definitely was pretty big, but I don't know if he was erect/semi or simply really huge flaccid.
One day I had stayed late after practice for some reason, and assumed everyone was long gone by the time I got in the locker room. I took a shower without my speedo on and popped a massive boner; I wasn't brave enough to risk jerking off, so I still had a full erection when I walked back into the locker area. Before I got halfway across the room, one of my older teammates walked in on me. I didn't have a towel or anything to cover up with, so I just kind of pretended nothing was up. He totally just stopped and stared, and I nervously hurried to my locker without saying a word. After I got my bag out and my boxers finally on, I saw out of the corner of my eye that he was now sitting on a bench and still looking at me (and the tent I was sporting). I turned around to continue getting dressed, and by the time I finished he had gone into the shower. I went right to the toilet stalls, dropped my pants, and rubbed one out while listening to sound of his running water. I left the jizz on the toilet seat and floor and hurried out of there, then jerked off two more times that night.
He never said anything to me the rest of the year, and I assume he never told anyone about seeing me hard.