I agree that if there are rules, it's much better to make them explicit instead of implicit. I also think your example makes sense, but I'm not sure it applies to larger groups or public forums. There are simply too many people to avoid discussing every topic that could offend someone. I'm not saying overt racism or homophobia is okay, but especially when it comes to humor it's almost unavoidable that it can hit too close to home for someone who sees it.

In public discussions there has to be some baseline ability by the reader to accept that they can be offended while the value of what was said to the rest of the audience may outweigh their own feelings. The implicit rules of what is acceptable to talk about, especially online, have been getting stricter to the point of being ridiculous in my experience.