What other "classes" of people do we have that are protected under law that are defined by behavior? Race and gender are not behaviors.
Actually, the first "protected class" ever defined by the US Supremes based on the 14th Amendment was US citizens of one state travelling to other states - there is a blanket ban on discriminating against them on that basis. See also Ward v. Maryland
1870 (still valid case law) and Saenz v. Roe
1999 which re-iterated what Ward