The key line is "extensive" (and also, of course, "always.") Imagine that out of a large population of Star-Bellied Sneeches and Sneeches With No Stars Upon Thars, a few marry and have Half-Starred Sneech children. Thereafter (for whatever reason) Half-Starred Sneeches tend to mate with each other, and, in part due to their greater resistance to the Great Whoville Plague, in due time grow to greater numbers than either the Star-Bellied or Blank-Belied populations. But this doesn't produce a (biologically, rather than culturally, mediated) "reason" to mix except insofar as the original Sneeches who formed multi-ethnic families had genes that made them less ethnocentric (and that this effect continues to be produced by this gene in the new environment etc.)