From the notes of the talk which others have cited: "If you don't know people who are different than you, you don't trust them. Think about this in the context of the politics around gay rights. The #1 predictor for how someone will side in issues of gay rights is whether or not they know someone who is gay."

Even if causality works in that direction, and you could eventually make bigots into better people by hanging around with them, who would want to spend their time that way?

We spend time with people we like and respect. If you regard that as a problem, what's the solution?