Not exactly on topic but close enuf. I think erections on a bus qualify as public, though perhaps only embarrassing with eyes wide open.
When I was in the Navy, I was stationed for awhile in San Diego and would commute each weekend back to my wife and young son in LA. Sometimes I'd hitchhike, but when flush I'd take the Greyhound. This Friday evening for the first leg I had a window seat near the rear to myself and quickly fell asleep. At Camp Pendleton I woke for a few seconds when a good-looking blond Marine my age took the aisle seat next to me. I could tell he was a jarhead by the distinctive high and tight crewcut they all wear.
A while later, I slowly came awake, immediately realizing I had a roaring hard on. I could sense I was leaking pre-cum. I was wearing shorts, a favorite of mine that showed off my muscular, hairy legs. I can't remember but I was either going commando or wearing government-issue white boxers. Whatever, a surreptitious glance revealed my super-slender little dick was noticeable tenting up and to the side.* I also felt my seat mate's leg pressed against my bare leg. A secretive glance showed he was also aroused, but his was the exact opposite of mine size-wise, much much larger and forming a long, thick pipe-like outline down his inner thigh. Won't hazard a guess to it's exact size but I'm sure he'd be a member in good standing here at LPSG..
I continued to feign sleep, occasional taking peeks at our comparative erections through my half-closed eyelashes. Our two cocks would make a spectacular entry in the "Big/Small Comparison Thread", probably breaking all records for number of "likes". The contrast in size made me even harder. Somehow I could tell he was faking sleep also; he'd occasionally press more firmly against my hairy leg, which I occasionally returned.
This went on until LA, where we both went our separate ways with no acknowledgement of the brief, veiled, vaguely homoerotic encounter. I'm sure, like myself, he considered himself totally straight, but it certainly was an erotic couple of hours.
* I legitimately claim 4" erect but that's super-bone pressed with the ruler nearly drawing blood. Non-bone-pressed -- and closer to what it looks like in the showers -- it's maybe 3.5" or a little over. Definitely small-looking on a 6'3", well-built, athletic dude.