This is true. Same thing with dogs. Pure bread dogs are infact slightly inbred, which is why the purest dog breeds (german shephards, rottweilers ect.) have genetic defects later in life such as blindness, hip dysplacia, ect.I'm not an expert in this but if you're in the same race chance are you might be 20th or 30th cousins or something. When humans migrated from Africa they migrated to different places and they became differented race cause they were isolated from other humans and are most closely related.
probably all bs but that's what I think happened
I don't even know why this is a question, who would be against different ethnicities getting married/having kids?