My first time discovering cruising was in a Chicago public library restroom in college back in 2017. I was truly caught off guard too: I just went in to pee, and the stalls here were all exposed and pretty close to each other. I started peeing, and my eyes drifted to the right and this Latino guy was stroking his uncut cock while looking at my own cock.
I think I froze for 2 seconds and realized what was happening. I finished peeing at that point, and I immediately got hard and just started stroking with him. I went from "tired, ready to go home" to "THIS IS SO FUCKING HOT HOLY SHIT". The bathroom was also very exposed so I got very nervous, and I pulled my pants up and walked out.
My heart was beating like crazy, and the guy started following me (which, in retrospect, was definitely kinda scary lol). I ended up going up a flight of stairs to another bathroom. He followed me there, and once we got in and saw nobody, we both whipped our cocks out. He grabbed my cock and kneeled down and started sucking it, which just sent me to the moon. (Especially given that I'm actually not the biggest fan of orals). I was so fucking nervous and so horned up, and I told him I was gonna cum. He kept sucking me and I shot my cum into his mouth.
I came to my senses and thanked him. He smiled and I immediately jolted out of the bathroom. I think my heart was still racing for the next half hour processing what happened.
So yeah... My first public restroom sex experience. Definitely got me hooked on cruising, and I can also tell some other stories where I actually got fucked in them too.