It sounds like you aren't the asshole, but your parents are.

Your daughter seems to show great restraint. If my grandparents said ONE GODDAMNED WORD about what I wore, who I dated, or any other matter, I'd tell them to fuck right off and never talk to me again until they managed to pull their heads out of their asses. In fact that's what I did, my end of the conversation being something like, "You can think what you want and say what you want, but I'm going to DO what I want. Accept it and we're cool. Can't wrap your mind around it? I'll be seeing you." In my case they shut up and learned to keep their outmoded opinions to themselves. Your mileage may vary.

"Without deviation from the norm, progress is not possible." Frank Zappa said that. Perhaps the "deviation" your daughter displays is just good old fashioned love for another human being, and seeing an interracial love affair is just what the doctor ordered for those whose buttholes seize up at the very thought of such a thing.