What could possibly turn you on in a public restroom?
One time my gay friend saw a guy diddling himself in the public restroom and was so turned on that he found his own stall to diddle himself in. I hope that answers your question.
That does work, but it still leaves the question of what turned the first guy on (because whatever it was, indirectly at least, is responsible for turning the second guy on). Even if it was the same reason, it can't go on forever; some guy at the beginning was turned on by something other than another guy. The question is
what?