My first experience was at a public indoor swimming pool when I was about 15. For some reason when I did get under the shower and out of my swim shorts it was nearly impossible for me not to get a boner. It always felt freeing to get rid of the wet and cold shorts and as soon as the warm water was hitting my naked body I started to grow.
Normally I would not get out of my shorts in public showers excactly for that reason. Even if it was a mens only shower. I just could not control it.
But some day I was in a swimming pool where there were no separated showers. There where a lot of walls build to create corners with shower heads behind it, so that you were kind of safe from onlookers. The showers were directly installed in a large hall, where everyone was passing through to their lockers.
That day I deceided that, that it was probably safe enough to use that shower and get rid of my shorts as well. I had a long way home to drive and I didn't want to get itchy because of the chlorinated water. So I picked a very tucked away shower spot behind a wall, so that no one passing through could see me, got naked and started showering. As usual I had a full erection as soon as the short were coming off.
After some moments under the shower, fully soaped, I suddelny heard a female voice: "Excuse me?"
I just had my hands on my head while soaping my hair and still sporting a raging boner. I opened my eyes and a girl around her late teens with a big smile on her face was standing right in front of me. I nearly had a heart attack and was trying to hide my erection with my hands and turning away from her. Totally surprised I asked her what she wants and she just said in a very smugly tone:
"I just wanted to ask you, if you could lend me some shampoo, because I forgot mine." All the while eyeing me up and down.
I told her that it was standing right over there on the wall and that she could just take it. I would not need it anymore.
But she persisted on taking just a little bit out of the bottle, so that she would not have to bring it back and squirted some of it in her open hand, while maintaining eye contact.
She closed the bottle again and through it casually back at me. I could not surpress the reflex to catch it. Only to become aware a second later, that I was again presenting my full hard-on to her.
Immediately I was dropping the bottle and clasping my hands over my cock againg.
Her grin had again grown wider and she just said: "Thank you very much ... for everything" and tourned around leaving me with my cock in my hands staring at her nice backside.
I would stay hard the rest of the way home.