Suppose a white person uses the phrase 'black hole' in the presence of a black person either in its literal cosmological meaning or in some objectively inoffensive metaphorical sense, and the black person takes offense and complains that the phrase is 'racially insensitive.' Actual case here. Compare that with a case in which a white person uses 'nigger' in the presence of a black person.
I have just marked out two ends of a semantic spectrum. 'Black hole' used either literally or in some not-too-loose analogy to the literal meaning — as in 'black hole' used to refer to a windowless office — cannot be taken by any rational person as a racial slur. For 'black' in 'black hole' has nothing to do with race. But 'nigger' used by a white person is a racial slur.
It is worth noting that I did not use 'nigger' in the immediately preceding sentence: I mentioned it. It is a standard distinction and an important one if you value clarity of thought.
How should we explain the difference between 'black hole' and 'nigger'? Here is a suggestion: the latter term is objectively offensive while the former is not: it is merely subjectively offensive. It is offensive only to some inappropriately sensitive and irrational people of minimal linguistic competence to whom no apology is owed and to whom no apology should be given. From this I infer that if I say something that you find offensive, that is not necessarily my fault. In many cases it is your fault. For example, if you do not know the meaning of 'black hole,' then you are either stupid or egregiously ill-informed, which fact is your problem, not mine.
What I have just written is boringly obvious. And yet it must be said in these politically correct times. Even squishy latte-sipping bien-pensant liberals should be able to wrap their pointy heads around it, the point is so simple.
Now for a more interesting case. Some blacks don't want to be called 'blacks.' They want to be called 'African-Americans.' But I find the latter term offensive to my conservative sensibilities, opposed as I am to the hyphenated American. Bang on the hyperlink to see my reasons. Now civility, though a virtue largely invisible to leftists, is prized by conservatives, and I want to be civil. Part of being civil is respecting others' feelings. But truth and reason trump feelings.
Therefore I refuse to call blacks 'African-Americans.' My refusal is principled and has nothing to with 'insensitivity' or racism. If a black person takes offense, then it is his fault and his problem.