I have read that Ahilya was not deceived when Indra came in guise of Sage Gautam to outrage her modesty. Being an advanced soul herself, she was able to see through the reality of the situation clearly that the man was not her husband but an impostor & Indra at that. Yet unfortunately, she did not resist. One would have expected her to put up a brave fight. By the way, there is a version that she was proud of her beauty and wanted to give chance to Indra willingly.
Bookmarks