Do people have anal sex at outdoor concerts?
Probably in the porta-johns.
There's a wonderful website (NSFW) called zombietime dot com that answers your question. The author lives (or lived) near San Francisco, and documented the activities of those attending the "Folsom Street Fair". Among other things. You might be surprised what goes on in large public gatherings of sodomites.
Do people have anal sex at outdoor concerts?
.........
On Bourbon Street they do. Right in the clubs and in front of the street audience on the balconies. Oral, too.