The only one better than that is "I can't be racist, I always have a laugh with XYZ" or "How can I be racist? I my cousin, twice removed uncle's dog's dad is black/ white/ green / blue / purple"
That's my favourite Rev - I have lots of black/asian/muslim/gay/one armed pygmy friends.
I live in a fairly large town, but up until my mid 20s (now mid 30s) lived in SE London, and I don't know any black, gay etc... people. I haven't gone out of my way not to meet different people, I just simply don't know any.
Yet I think I'm fairly unique on this site as it strikes me everybody's best friend is either black, gay or a member of some other minority.
It is not really an oxymoron. Calling such an expression an oxymoron is sometimes done in order to disparage its use, by drawing attention to a perceived inherent contradiction and thus claiming it to be nonsensical.
The �but� element is automatically assumed to be the part which joins two simple sentences to make a compound sentence. Much like the word �however�. When used in a sentence it indicates that the relationship between the two independent clauses is one of contrast or opposition. Some could view it as an aside, but that would require a greater grasp of English grammar and written language.
So naturally one would assume that the opening line is leading to compound sentence and will thus be followed by a contradictory comment, asserting therefore that the person may well harbour, innocently or otherwise, some elements of racist emotion.