I think part of the challenge is that people are conditioned by media to view "sexism" or "racism" only in the grossest extremes, as things that are only done by truly evil people. For example: "sexism" as the boss who gropes his female subordinates and proposes sexual services in exchange for special treatment or promotions; "racism" as the cross-burning Klansmen or curb-stomping skinheads.
As such, admitting to "a little" sexism or "a little" racism is viewed by many as a one way slippery slope of perception where their friends and colleagues will soon be viewing them as those aforementioned examples of undiluted evil.
Additionally, from a tactical / practical point of view, it may be unwise to admit anything in front of HR, which generally exists primarily to protect the company's ass from liability, and only incidentally to protect the employees. No one wants to be written up or sent to sensitivity training.
As such, admitting to "a little" sexism or "a little" racism is viewed by many as a one way slippery slope of perception where their friends and colleagues will soon be viewing them as those aforementioned examples of undiluted evil.
Additionally, from a tactical / practical point of view, it may be unwise to admit anything in front of HR, which generally exists primarily to protect the company's ass from liability, and only incidentally to protect the employees. No one wants to be written up or sent to sensitivity training.