Of course genetic mutations are a pretty common thing, but some are minor (like your red hair example), some are “WTF? How did that happen?” mutations, and some are straight up deadly like the various forms of cancer.
Women being XX is a classification (?) we’ve all agreed on because like 99.8% of people with XX chromosomes are women. Mutations are literally something out of the ordinary that shouldn’t happen.* A woman shouldn’t have XY chromosomes because almost every woman in history that we know of has XX chromosomes*.