When you don't know who or what kind of person you're talking to, the opportunity for making a faux pas increases. And, when you talk on a forum, that opportunity increases many fold, since your comments are open to every member.... So you have to be careful how you say things. That person (or persons) on the other end may not always share your politics, religious beliefs, or race. Assuming that everyone just naturally agrees with us can cause some awkward moments like the time I was sitting with a co-worker having coffee.... So it pays, all the way round, to take a moment and consider how your posts might be taken. That "liberal pacifist bleeding heart who doesn't care about his country" just might turn out to be a decorated disabled veteran.
The short version - Don't use slang names for groups of people ... This isn't being restrictive so much as extending a common courtesy.