I believe the argument is that white nationalists "chose" their political beliefs, so it is the right of anyone to chose to not do business with them. Whereas a gay couple does not choose their sexuality (yes, this has been well established in the annals of biology and psychology), so they cannot be discriminated against. Indeed, any fundamental aspect of one's identity (race, sex, etc) cannot be grounds for discrimination in a just and fair society. 

That's not even getting into the public safety aspects of it.