"Why should I pay for taxes to support single mothers just so black men don't have to pay child support?"
Now, I think we should still encourage them to ask those kinds of questions, because I believe in an open and liberal society and that the best reaction to poor thinking is to address it directly, that the answer to speech I don't like is more speech. But it's pretty easy for some really racist opinions about others to hide behind questions like that.
Of course, there are also some uncomfortable truths in those questions which cause us to silence them, such as -- "Are the disparaties we see in racial groups purely the effect of lingering racial bias, or is there a modern cultural component that happens to align with racial groups and perpetuates the inequality?" (And in this case -- the more general, is there a disparity in the fatherhood rates that correlates with racial grouping?)
Ed: I think the people downvoting proved someone upthread's point -- I literally provided an example I've heard of such a question and got downvoted for providing facts to a discussion.