Saying "you can't legislate stupidity out of humans" is essentially arguing that anti-discrimination laws are a waste of time. In fact, it's saying a whole raft of laws are a waste of time. Why have any legislation at all? After all, "you can't legislate the stupidity out of humans," so what's the point?

Of course, the problem here is the argument is a straw man. No one is trying to "legislate stupidity out of humans;" what we're talking about is preventing people from behaving with stupidity and prejudice, however stupid and prejudiced they may remain underneath. People are stupid. And guess what? Anti-discrimination laws work, have been working since they were passed (back then the argument was "You can't legislate morality," made by Barry Goldwater and the Dixiecrats in defense of Jim Crow laws). If "you can't legislate the stupidity out of humans," yet anti-discrimination laws work....?