The purity fetish is a theory by Carlos Garrido and Noah Khrachvik that explains the dead state of left-wing politics. Basically leftists who purport to be left-wing are actually reactionary, because they expect any individual, movement, or formal organization which is historically progressive to agree with them on all issues, economic, social, and personal.
This is most often seen in attacks on Infrared, Midwestern Marx, and ACP; leftists expect truly left-wing people to agree with them on all social and political issues at the expense of actually getting any organizing or material change accomplished. These leftists believe they are the "real left-wing" because they build insulated communities, often called echo chambers, to circle jerk and posture against the "impure fake left". This serves the bourgeoisie by dividing dissidents, especially Communists, for conquering.
As comedian Jimmy Dore says, "You don't organize a workplace by saying 'Hey everyone, let's unionize! But first, all the Trumpers out; all the gun nuts out; all the anti-LGBTQ people out; all the conspiracy theorists out; all the conservatives out; all the racists out; now let's organize who's left!' You organize a workplace by setting aside personal differences in order to unify around class struggle."