Yeah, I can't think of anywhere in the US this would overlap with a protected class - so long as it was consistently applied. If someone says they discriminate based on your sign, it would warrant a very close look to be sure they aren't actually only checking the birthdates of people of certain protected categories.. this sort of red herring discrimination does happen, but if it was truly and consistently based on someone's sign, that would likely be defensible everywhere in the US.