This is a dated term, usually used for humorous effect. It uses the word fair in the sense of “physically attractive”; because it refers to a woman in terms of her appearance, it is sometimes perceived as demeaning.
British Dictionary definitions for fair sex
fair sex
noun
1.
the fair sex, women collectively
Idioms and Phrases with fair sex
fair sex
Girls or women, as in Many women would object to being called the fair sex nowadays. This euphemism uses fair in the sense of “physically beautiful” and is probably dying out. [ Mid-1600s ]