Standing out in a crowd.

The only reasons for why he's doing this that I can come up with is that either he's feeling randy and he's very brave (and pissed? - though his standing straight doesn't suggest it) - or it's some sort of a dare or forfeit (in either case it being fortuitous that he's evidently getting a great kick out of it, possibly on the very brink of 'eruption' without even touching). Just being naked by itself would have attracted some attention, but so publicly sporting a boner has made him a crowd-puller with several cameras out. Hope he achieves whatever it is he wants before the cops come and put a spoilsporting end to his 'fun' - or whatever it is.