It’s a difficult thing to explain, because the kind of person who says “I’m only into to white guys — sorry that’s just my preference” is operating from the fundamental assumption that one’s personal preferences are the product of independent and original thought. Nothing could be further from the truth.Our personal preferences aren’t personal. They don’t spring forth from some internal source. We absorb them from the external environment, and that environment is racist as fuck. Our concepts of beauty, masculinity, femininity, and social status — hell, the very idea of race itself — these are all just things that get poured into our developing brains during our enculturation.
What we ultimately find attractive isn’t up to us. It never was. Sure, there’s some fundamental biology as to what turns us on, but the things we’re “into” are mostly just layers upon layers of social constructs. The problem with explaining this to people (especially small-minded racists) is that no one likes to think of themselves as being culturally programmed.
People want to believe that they’re autonomous individuals. They’re not. None of us are. We’re all products of our culture — a culture steeped in systemic, historic racism — and that’s what you first have to explain to a person before they’ll ever come around to understanding how their personal preferences can be racist without them ever having realized it.