The don't take much offense over the word guido, it's the fact that they are supposed to be representative of Italians. I have seen people like this come into my work (alpha male jack-offs of every nationality) and they all want their clothes 3 sizes to small. It's like emo with a tan.
As some who is at least partially Italian I only get annoyed by the fact that they try to make the stereotype the norm. It's kinda like the people who say all southerners are illiterate racists straight outta Deliverance.