I've run into a lot of discussions this week (in various places, mind) that have exploded into arguments between "oversensitive humorless whiners" and "uncaring ___ist douchebags."
A lot of what constitutes 'funny' is subjective, and depends heavily on context and handling. Some subjects, to some people, will never be funny. The most brilliant and well-executed blonde joke is still going to offend one or more people (who may or may not be blonde themselves) simply because it is a blonde joke.
Tone matters. Many jokes about race, religion, etc. are funny (or not funny, depending on audience) because they play to stereotypes. Just for the sake of argument, let's say I tell a joke involving a stereotypical Irishman. I want my audience to think about it and go "yeah, I can see [insert Irish friend/relative/whatever] doing that." I don't want them to think "he's being an asshole and picking on the Irish!" If they do, I've failed to make the joke funny.
There's a difference between "seeing the humor in something" and "finding something funny." I've heard some truly horrendous racist jokes. I found them ugly and offensive. Doesn't mean I can't see how they were meant to be funny.
Other people laughing at a joke you find offensive doesn't automatically make them assholes. Nor does other people being offended by something you found funny make them overly serious (or, for that matter, make you an automatic asshole). Challenging another person's right to be offended does make you an asshole. If someone thinks the joke that your friend just told is the most profane thing ever to be uttered by human lips, chances are they have a good reason. Dismissing that reason is not going to make them less offended. And explaining to people why they are horrible people for finding something funny goes over just about as well. A simple "This is why I find this joke distasteful" is generally enough to get people thinking.
Always keep in mind that you might be a bigot. It happens to all of us. Especially the people who think it doesn't. Doesn't make you a bad person until and unless you refuse to correct it.
In short, one person's humor is another person's hate speech. And while stereotypes can be funny if done properly, it's still a bad idea to tell black jokes at an NAACP rally.
EDIT: This was done quickly, and may not be as coherent as it should be. Criticisms, suggestions, and the like are welcome and encouraged. This is a blog, after all. ;)