There is one really important point to keep in mind, when someone is offended about something someone else says or writes that is their decision and it is their problem not the one who did the offending.
I personally don't generally use the term "retard" I also don't generally put slurs of any kind in my code. However I do take issue when someone says that I can't.
"Whoever is the stupid fucking idiot who made this dimwitted decision should go get their brain checked."
That's no less offensive then using the word retard and it's anyones right to down vote the comment on that basis or to just ignore me. Thus my point has been made.