I think I know why this discussion is such a problem.
So, by definition:
Race:
A socially constructed category of identification based on physical characteristics, ancestry, historical affiliation, or shared culture.
So, by refusing to date a certain race, you can be refusing any of these:
physical characteristics, ancestry, historical affiliation, or shared culture.
So you can have preference on physical characterisics. You can find some races physically unattractive based on certain characteristics (ex. Skin color, facial features, etc) which define their race. Therefore, you can prefer just not to date that race.
Which is fine, that doesnt make you racist. You cannot control who youre attracted to. Same with sexuality. People cant control who they find attractive.
Now, refusing to date someone based on the other parts of the definition, which generally have nothing to do with who the person actually is:
ancestry, historical affiliation, or shared culture
If you dont date someone based on those. Id say yes, thats racist.