sex vs femaleness

sex and femaleness both are nouns.

Word	Noun	Adjective	Verb	Adverb
sex	Yes	No	Yes	No
femaleness	Yes	No	No	No

As nouns, femaleness is a hyponym of sex; that is, femaleness is a word with a more specific, narrower meaning than sex:

sex: the properties that distinguish organisms on the basis of their reproductive roles
femaleness: the properties characteristic of the female sex

Other hyponyms of sex include maleness, masculinity, androgyny, bisexuality, hermaphroditism, feminineness.

sex (noun)	femaleness (noun)
activities associated with sexual intercourse	the properties characteristic of the female sex
the properties that distinguish organisms on the basis of their reproductive roles
all of the feelings resulting from the urge to gratify sexual impulses
either of the two categories (male or female) into which most organisms are divided

sex (verb)	femaleness (verb)
tell the sex (of young chickens)
to evoke sexual feelings