There are different classes of things that are "known" in society. What class something is in depends on a lot of different things, and funny things happen when they change class. There's a fairy tale about it: The Emperor Wears No Clothes. In that one, a kid makes unavoidable what everyone is thinking but afraid to say. Before the utterance, it was unthinkable to say it. After, it was unthinkable not to.

You can see this in play right now - there's that dynamic about the Weinstein trash fire, for instance. I think that was part of what was going on with the poster. It was shocking enough to be an instant conversation topic ("went viral"), and greased the rails a bit for people to admit to their peer groups that they opposed the war, too.