I don't think it's racist to have a preference to date people of your own race and most people would probably find that reasonable I'd imagine. Excluding the idea of ever dating someone from a race outside yours of even one specific race sounds pretty prejudiced. That said, I'm willing to bet in 99% of cases that, put to the test, nobody would stick to that stance. If OP's friend for instance met the girl of his dreams, perfect for him in every way, but she just happened to be black, my bet is he would change his tune pretty fucking fast. If he didn't at that point, that is absolutely racist.