Moral contextualizing is the Saul Alinsky-esque technique of applying a context to behaviors in order to assess their morality—in particular, to assess the behaviors of men in the past within the context of today’s views of morality rather than the views extant at the time.
Paul Isaac had some thoughts on the matter in a letter to the editor of The Wall Street Journal:
The real issue is “contextualization” per se, which seems to be an aggressive technique to create false dialectics against historical straw men and to demand an implicit intellectual monopoly of the would-be contextualizers’ perspective on the defined hobbyhorse as the moral alternative, thereby defining other perspectives as both outdated and immoral.
That’s a big part of it. Another part of the thing is the hypocrisy of the moral equivalence that these contextualizers assert in order to claim the holy superiority of their chosen, and debate-forbidden, positions.