IMO if they are allowed to force a "slut walk" to be public. Anyone is able to force their opinions also. He didnt tell anyone to suck his snake, he didnt rape anyone. All he did was flash them. That was no reason to result in the violent actions it looks like the females were about to do. He is not forcing anyone to look at his snake. Besides, if anyone is traumatized that bad by a snake, they shouldnt be in public.
Besides, any gatherings of people in any group, will normally have some of the opposite group come to taunt them. Welcome to the normal world, this stuff happens all the time.
On the first subject, they forced their opinions on him and everyone else around them. A parade/walk is ignorant itself. That means they are moving their location to spread whatever they are there for. IF they were in a small park doing a rally, it would be one thing. But practically walking around going "we were victimized" doesnt solve a single thing. Besides pushing their issue on any nearby people. If they wanted anything to be done, they would have sued the rapist and/or pressed charges.
Edit: Besides, it only takes a couple people to turn something like that into a riot. When people start grouping in a crowd with a large chunk hiding their faces, mob mentality kicks in.