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Everyone marries outside their "race" because an ancient god of purity of blood cursed the world with their dying breath that all who would be pure in their eyes will instead become horribly inbred.
Why, you ask? The god was driven to madness and death when a mighty, conquering empire their chosen people built grew too big, and all the other gods and races banded together to stop them from taking over the world and destroying all who were not them.
The alliance of gods and races succeeded, and the god decided that if they couldn't, then no one ever again shall claim dominion over a pure race and lead them to victory like they would have, either.
In the aeons since, it was forgotten which, if any of the currently existing races was the chosen people of this god.