I'm trying to find a good metaphor to use to get people who try and dismiss casual *ist language as "not a big deal."

I'm thinking papercuts. They're small, but they hurt a lot. And no one wants to get lots of papercuts every day. The casual everyday non-intentional *ist language is sort of like those - so if you're a member of a non-privileged group, you get hundreds or thousands of emotional papercuts every day.