This is either blatantly racist or extremely silly.
Encouraging diversity isn't about encouraging (or, I shudder to think discouraging) inter-racial relationships, it's about promoting tolerance, love, and understanding between all peoples. If that tolerance, love and understand leads to a black man and white woman falling in love and having babies, how does that "harm" diversity? In fact, I would say we have MORE diversity now as a result of interracial relationships - whole new "races" of people with unique combination of genes. Mulattoes (like me) aren't the only hybrid anymore.
I don't know what you mean by "maintain the purity of the races" but it sounds eerily Final-Solution-like to me. Did you have a small black mustache in another life?
Diversity is great. Freedom to love and have babies with anyone you want is also great.
I sincerely doubt that "everyone will look the same" in the future, as a simple matter of genetic probability. The children of any two people of differing race may have very different skin color, hair color, facial features, etc. I have 9 cousins, all with the same mother and father, and they are all different shades of brown, from just a bit tan to deep chocolate.