Realists and honest people acknowledge that human races exist and are different. No one is saying racial or even species boundaries are "rigid." That's a straw man. No serious person ever said that they are. The Pacific Ocean gradually changes into the Indian Ocean and the Atlantic Ocean and the Gulf of Mexico and the Mississippi River. And the Mississippi River gradually connects to a tiny spring in Itasca, Minnesota. But no sane person would assert that, because of that, the spring behind my house is the same as the Pacific Ocean, or that the Pacific Ocean does not exist.