I agree, people can be conditioned to accept or reject other races. It is not at all clear to what degree race blending is desirable. You mention that it might be biologically sound to race mix because of the rich gene pool. On one particular scale, that is true and reflects the mathematical underpinnings of diversity. However, if that behavior is looked at at a longer scale, and the behavior and resultant gene pool is allowed to reach homeostasis, the richness of the gene pool collapses into an indistinct average and diversity collapses with it.

A more robust diversity preserves and isolates a portion of each distinct branch while allowing additional diversity with some mixing. Each genetically distinct branch represents thousands to hundreds of thousands of years of vested evolution. Each branch may be best thought of as a resource for the human race that is a Darwinistic hedge against a particular set of catastrophe which only they hold the key to survival for. Policies or trends that diminish the maximum diversity , particularly that infringe on the distinctiveness of the sub-pools, are provably detrimental to the robustness of the gene pool in the long term.

Wiping out real diversity in service of multiculturalism is a poor trade to make its proponents feel like they are virtuous. Even if race-mixing via multiculturalism succeeds completely in its goal and a brown world is achieved, people will still kill people. Instead of skin color, it will be for a random reason like how they style their hair different.

The real problem is that the world is filling up and homogenizing. All of the personal and national identity crisisi that are boiling to the surface now are part of a complex instinct system that nature has provided us to counter over-homogenizing our gene and meme pools. We need good boundaries so we don't have to resort to bad boundaries like wars, hatred and killing. They must protect viable pools of distinctiveness while not encroaching on other pools.