Getting naked w/ friends is a favorite turn-on for me, so I have several stories. Here's one that comes to mind. I had just arrived in the gym locker room to get changed for my training session, when I heard my trainer (a hot, fit, straight Latin guy) talking to someone in the back section of the locker room.
I knew he would have to pass by my area on the way out of the locker room, so I quickly took off and put away my shoes, socks, and pants, and unbuttoned my dress shirt. (I had no undershirt on) Right when I heard him and his friend enter the front section of the gym, I dropped my boxers. I then turned around (with dick fully exposed between the shirt tails of my unbuttoned shirt) to drop my boxers in my bag, when...surprise (not really). At this point, fully facing him, we exchanged hello's "see you in a few minutes", etc., then I just proceeded, unfazed, to take off and hang my shirt, and put on my gym clothes.
During the entire workout I was fighting off getting an erection as I was so turned on by knowing that he had just seen me my dick. I had a great j/o in a shower stall after the workout.