Imagine basing your entire perception of a person based on a single group identity that they have, it turns a 3D person into 2D cutouts of themselves and reduces their opinions and belief's to the group identity and not the persons identity.
(It also dehumanizes them making it trivial to justify uncivil behavior.)
Never judge a person under normal circumstances based on their group as you don't know their reason for their belief. (Provided that their not a extremist.)
Think for yourself and be careful to understand another's POV before making a judgment about them as a person.