Stopping fetuses from developing Down’s Syndrome in my opinion isn’t unethical because it will genuinely improved their quality of life. They will live longer lives, have fewer health problems, etc. The slippery slope however was pretty well covered in the film Gattica in which people not only start requesting designer children but the world becomes a dystopian utopia where the genetically perfected are unfairly favored as the ruling class while the genetically unmodified become relegated to the worker/slave class.