fair sex vs social class

fair sex and social class both are nouns.

Word	Noun	Adjective	Verb	Adverb
fair sex	Yes	No	No	No
social class	Yes	No	No	No

As nouns, social class is a hypernym of fair sex; that is, social class is a word with a broader meaning than fair sex:

fair sex: women as a class
social class: people having the same social, economic, or educational status

Other hypernyms of fair sex include class, socio-economic class, stratum.

fair sex (noun)	social class (noun)
women as a class	people having the same social, economic, or educational status