Undercover Black Man has a nice post outlining the genetic advantages to race-mixing, something I've always thought should be obvious to anyone who knows anything about genetics. You don't even need to know about genetics. Just look at the hereditary problems in close-knit and inbred populations. The post details quite a few of those. It's a nice, inconvenient fact for those who think race-mixing is unnatural. Even aside from the difficulty such views face in identifying exactly which populations are the races that can't be mixed, it does seem as if nature prefers combinations of genes that are less closely-related than combinations that are too closely-related.
I do think, however, that it's worth acknowledging that some effects of combining the DNA of very distantly related people could be more harmful. If a trait requires gene coordination from both parents, and the coordination requires more closely-related DNA, then such a crossbreeding could lead to a loss of those kinds of traits, even if it's more likely to preserve traits one of the populations has lost (because those traits are usually simpler).
So it's not purely a matter of race-mixing being healthier and monoracial reproduction being less healthy. There are benefits and disadvantages either way. But the most common opposition to race-mixing in the U.S. context is the racist idea that white genes shouldn't be polluted with black genes, and blacks and whites in the U.S. at this point are much more closely related than most other interracial pairings, largely due to race-mixing in the past (ironically caused mostly by white slaveowners raping or seducing their slaves). Given that, I would expect these negative effects to be significantly reduced in black-white pairings than would have been true in the time of slavery.
So I do think the conclusion is correct. If anything, interracial relationships are at least in one respect more natural than same-race pairings.