It may be legal to post such a sign but it isn't legal to act on it in a discriminatory manner as considered by the Constitution and the courts. But you all already know that; you are simply whining about your feelings and wishing the world catered to them. And wasting everyone's time.
We don't have a constitution in the UK. And I'm not whining. Offering information for others to read is not discriminatory either. Nor is it a waste of time.