There was one time, heeeeesh...
I was at a pool wearing swim trunks that were not too small, but rather too large, for me. They were sliding down just a touch, some of my hair was showing, and a girl I found to be a haute chic pointed it out. I looked down, and popped one right then and there, and it was at full power in about a second. You'd think that'd keep the trunks on, wouldn't you?
There was another time, I had a physical (there are so many physical erection stories out there, I know...) and the doctor was of course female, and somewhat attractive, but I wouldn't seriously consider dating her. When she asked me to cough, the sensation of my gonads jumping in her hand gave me an instant and rock-hard erection. I apologized, stuttering, but she replied, "No, no, this is fine--this is good. This'll give us an opportunity to head off any problems at the pass you might have in this department." She didn't fondle it sexually, but she held it, shifted it around with two fingers, and observed it pulse. Then she gave it a sharp thwack with two fingers and it bounced around. She blushed slightly, and said, "That usually makes it go down. Maybe I didn't hit the right nerve. Mind if I try it again?"
Well, if she'd tried it again, I suspect it might have necessitated a change of scrubs on her part, so I told her I'd just let it go down naturally. She blushed and smiled, almost as embarrassed as I was, it seems, and told me she'd get the nurse to wrap things up, she needed to tend to another case. I decided to let the monster fall asleep before I put my clothes back on, since I was wearing briefs and it wouldn't have done my erection any favors. So when the nurse came in, she saw my member at Power Level 9000, tittered, and offered to turn while I 'concealed the beast'.
The doc wasn't embarrassing. The nurse...really was.