Those who worship race hate the mixed couples most of all. Once the Mutt-baby is born, they have to deal with it, honorably or not, but, if they can discourage the pairing before the fact...then the purebreds can more effectively prevent their own eventual waning
Mutts were created first by adventurers, travelers, traders, and others that roamed the planet, when it was still hard to roam. Race has been one of the most important causes of war and violence, second possibly to only religion, and mutts, besides often being the offspring of war, are the worlds longest and most violently oppressed class.
It is because of these influences, in combination with Hybrid Vigor (AKA, heterosis, as bonobo points out) that has made the 'race' of Mutts the strongest and will one day make us the most prevalence race.
It is through hybridization that all genes will
ultimately be saved and expressed.