I'm not sure about this reasoning. I think it would be quite possible to teach Johnny that the naked bike riders are nit at all like the scout leader getting his penis out. Just like kids can understand it's ok to be naked in the men's change room, or at a sauna, but that some kinds of nudity are not ok.
The origins of these rides in my memory were that it was about being a bit edgy and pushing boundaries in order to get attention for a cause. Which in some ways is a little old fashioned at this point in time when we can turn on our computer and see 10 naked people screwing any time we want.
However, what the people who want to be a bit edgy seem to miss is there is often a reason for a social taboo. I think there have always been some people who like an excuse to get a bit of attention when they are naked, there were always girls when I was a student who liked to do the naked bike ride and I think that's what it was about. It's not a great thing to encourage. And then there are the sexual exhibitionists, and I think it's very appealing to those people as well. Which again, it is really better not to give those people that kind of attention.