Gender and race are protected classes in certain situations and that provides them certain rights under the law. Right now, union members are not a protected class, and so in order for them to have protection against discrimination, that has to be done at the legislative level, usually the state level.
I am saying I am inclined to agree that people should not be required to join a union to work at a particular business. On the other hand, if that is the case, then neither should those that choose to do so suffer in their own careers for having made the choice.