It could be said that mating within one's race is somewhat more "natural" than mating outside. Things like pheromones and genetic memory drive people to naturally seek those who are somewhat similar in this regard.
This is simply descriptive, not prescriptive. There's nothing inherently wrong in race mixing, generally speaking. Though the results can be... shall we say, mixed, in terms of health and whatnot, good or bad. Bringing genetic diversity into a genepool can improve it to some degree, or the opposite. It really depends.
Some people get together for the wrong reasons. Imo, no one should have children together unless they are genuinely in love. Interracial couples based on love are great, but when its based largely on some exotic fetish, that's not so great.
My two cents.