If whites only have kids with boacks and blacks only have kids with whites that doesn't result in an all white generation, it results in an all mulatto generation
That wasn't really a retort to BoredVirulence, as that would just result in the destruction of a few dozen races to produce 1
In addition to Loud Whisper's fine comment, you're assuming equal proportions. Its never equal proportions. Hence why we use terms such as minority and majority (actually, do we ever talk about the majority? Or do we just universally call them white?).
You're assuming that its the destruction of all that currently exist to create something new. I'm saying that the majority will be slightly modified culturally, but mostly intact where the minorities are gone. "Little Mexico" and "Chinatown" regions exist not because people are racist, but out of cultural pride. Its like saying we assimilate Mexican people, culture and country, into the US and getting US with Taco Bell. Of course it would swing based on population, so we'd all just be Chinese and America would not exist, nor Mexico, Africa, or the various cultures of Europe.
Edit:
While I like my dramatic imagery of exploding people to point out that the conservation of people doesn't make it less of a crime, I think Loud Whisper's comment does a better job of explaining how mixing diversity and assimilation can be a destruction. Worth repeating.
But the group isn't any more destroyed than sugar is destroyed when it is dissolved in water
Mix all the colours in paint together and you get a grey, colourless mass.
Dissolve crystals in water and you've destroyed the crystals