Why are references to gender usually not considered sexist while references to race are seen as racist?

Example:

"That Mexican cashier doesn't know anything." Racist? Yes, because what does her being Mexican have anything to do with it?

"She doesn't know anything." Sexist? Not by most standards. Yet similarly, what does her gender have to do with not knowing anything?

Are seemingly harmless references to gender soon going to be a taboo?