It's not as simple as preference or "racism," which doesn't mean the same thing when it comes to people who have been and are racially oppressed. We don't live in a neutral, ahistorical society. Since the 1600s in what's now the US & the 1500s in the Americas (Canada to Argentina) forms of slavery, racial exclusion, segregation, racial domination, White supremacy, racism, etc. have existed. They existed & exist in every country in the Americas. Moreover, whiteness is centered in the US, still, and across the Americas. You see it in Brazil, you see it in Colombia, you see in Dominican Republic, you see it Cuba (which had a revolution that banned racism), etc. Even in predominantly Black countries, you see how colonalism, imperialism and white supremacy operate. So when a Black person says he/she/they does not want to be in a relationship with a White person, some level of thinking about this history and how it still colors this society and others might factor in. Joseph Beam famous wrote "Black men loving Black men is the revolutionary act." He didn't mean that Black men had to exclusively be in relationships with each other, but rather that this was and remains a radical act in a society in which, despite our incomparable gains, we are still oppressed as Black and as gay men. (This goes for LGBTQ people in general.)