That's valid. That example would be a form of cultural prejudice that affects everyone in that subculture. For that reason it could be called systemic, I suppose. The general idea of "systemic" is that it has to do with things that people do not directly see or things that are taken as being part of the normal fabric of society. This could be a borderline case between being a tribal prejudice vs. being something we could call systemic (applying to institutions and practices across a whole society rather than only to one part of it). Either way, it would seem to be irrational as a norm to discourage people from marrying outside of their race, and it could cause situations that are unjust (at the very least, unjust in the interpersonal sense of bringing unfounded moral disapproval).