As Martin NiemÃller sagely said:
First they came for the Socialists, and I did not speak out - Because I was not a Socialist.
Then they came for the Trade Unionists, and I did not speak out - Because I was not a Trade Unionist.
Then they came for the Jews, and I did not speak out - Because I was not a Jew.
Then they came for me - and there was no one left to speak for me.
Political correctness* stems from a perfectly reasonable idea: be nice to other people.
But as the Founding Fathers wisely intuited 240 years ago, to INSIST on that itself is at root a sort of social tyranny, which indeed then opens the slippery-slope question "according to whom?"
A multicultural society CANNOT function in which everyone has to constantly try to anticipate everyone else's triggers.
The only reasonable solution is a general promotion of freedom of speech and internalizing the idea that offense is self-created. This isn't to say people shouldn't be offended; in my view much of the progress of humanity has stemmed from people being offended at something or another. They certainly have the right to their offense. But when this offense fuels actions that are then designed to constrain other peoples' right to their own freedom of speech - there a line is crossed, and the corrosion of free speech begins.
(And for the pedants, yes, I'm aware that the Constitutional provision about free speech only applies to the behavior of the Federal government; I'm speaking more broadly in terms of cultural values.)
*the real comedy is that there are still people who ardently insist there IS "no such thing" (http://www.dailykos.com/story/2009/9/24/781372/-)