There is some truth to that--people tend to discriminate against people who are different from them, and the most obvious marker of difference is phenotype. From this perspective, a black/white marriage is a blow against racism, and our history of white supremacy, because it produces kids who presumably don't represent the phenotypical extreme of blackness or whiteness. The hope is that one day, we'll all be beige hence rendering racism inoperative, hence the "Beige Theory" of fighting racism.
I think to believe that requires a misunderstanding of humanity, history, and white supremacy. I'll start where I'm weakest. I don't know how the great St. Clair Drake's book, Black Folk Here And There has held up, and perhaps its conclusions are outdated. But as I recall, one of the books most profound arguments was that while prejudice and stereotyping based on--but not limited to--color is pretty natural, white supremacy is not. There is a difference between thinking black people--or any people who are different than you-- aren't that bright, and thinking that any black man should be lynched for having sex with a white woman.
More to the point, there is a difference in between thinking that blacks
are animalistic and thinking that blacks are, by edict of God and science, bound to a life of slavery and second-class citizenship. For starters, while we've become accustomed to thinking of animal stereotypes in relation to blacks, it's actually pretty common theme of group vs. group prejudice, black or not. Now, one may follow from another, but it doesn't necessarily. And when it does follow, it's often about something more than naked phenotypical hatred.
Thus the point of slavery, and Jim Crow, was not to express a naked hatred of someone who looked different than you--it was to create a permanent class of free labor. In the early days of this country, you find racism, but you also find black slaves and white indentured servants running away together. You find them joining
Daniel Shays Nathaniel Bacon in rebellion (and presumably buying into his racism against Native Americans.)
You find them fucking and marrying, as humans are wont to do. And when Virginia decides to outlaw interracial unions, you find them organizing against the law. These people looked very different from each other, and while there was considerable racism, they did not, en masse, look at each other the way their kids would look at each other a century or two later. Color prejudice was the means of systemic racism in America, but it wasn't the end.
It may seem, at first glance like the ancient marital beef between blacks and whites extends from how different we look. But it's so much more complicated. And humans being humans, even if we were all beige, we'd find some way to discriminate. Assuming that we can destroy whiteness or blackness, assumes that these are actual, tangible things which can't be redefined, refitted and reformed. But history says otherwise, and I'd argue that should we ever become one race, we'd basically create new ones.
Circling back, it's true that the interracial couple is striking a blow against white racism--but not because they're creating a beige kid. They're striking a blow because they're thinking more about their own individuality, their own humanity, than about convention. We can all applaud that--and while applauding it, understand that the notion of fucking our way out of racism, presumes too much of our good will, and too little of our imagination.