I agree with Phillip Pullman when he says that "nobody has a right to never be offended." and that people should be free to say things that may be offensive, especially on their own time.

The simple fact that you've been offended isn't reason to go to HR. You go to HR if you're being harassed or if people are actively creating a hostile work environment. It correlates with being offended, but it's not exactly the same thing.

If people were harassing you about your MJ legalization stance (or anything, for that matter), then it probably would be appropriate to go to HR about.

When does it end? Right there. Easy.

As far as "silencing" goes, I'm assuming that the commenter above isn't trolling and honestly doesn't know that what he's saying is profoundly insensitive. In an intellectually honest community, people can have discussions around what would and wouldn't be considered insensitive, especially when that's the basic context of the discussion.