Literally, hundreds of times .. When I used to live in London, there was a toilet under Charring X station that was busy AF, especially on a weekend towards last trains - I'd go there a ton on my way home from being out on the town and because it was a huge very public loo - never thought to do much more than just go the the middle of the set of three open urinals and take a piss and leave .. then I noticed on one occasion that there was a few blokes at the further ends of the urinals discreetly (or not) having a bit of a wank - which I thought was a one off given I'd never noticed it before. Well .. now that I knew, it was like the magic platform for hogwarts .. this happened literally every single time I went to the station day or night. Tons of blokes getting their wang out for a bate - plenty of blowies and every now and then.. people would shuffle off to the cubicles for a fuck .. What I learned is .. you just need to be aware rather than take a piss, look straight ahead and go.
At the loo's attached to the food court near where I work in town - there are always business men jerking off in the stalls looking for a bit of under the divider attention - you can see the shadow of jerking hands on the floor .. don't think i have ever been in there where there hasn't been at least a couple of guys going for it.
Outside of that - there is still a reasonably active cruising scene here in Melbourne so, plenty of nature reserves where men go to get blown or blow - tons of men standing around in the reserve with their dick out hoping for a gobby.
There is also a nude "beach" near a river that is about 20 minutes away from my house that I go to every now and then in summer ..one section is pretty much mixed sex and just nude - a little bit further on is a mostly gay section that you can sunbake nude in but is also cruisy as hell where guys just have full blown sex on the river bank .. which is fine till a the kayakers swing past.