I sincerely doubt that "anti-vaxxer" is a protected class. I'm guessing that some suits will be filed, and those filing the suit will have to come up with membership in a protected class, otherwise the suit will be tossed for lack of standing. The suits that don't get tossed will be decided on the basis of "are you really in that protected class". That's a dual-edged sword. Make it super-easy to claim religious group membership? You've just made it super-easy for peace churches to really go after registering for selective service.