sex vs sex activity

sex and sex activity both are nouns.

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

As nouns, sex and sex activity are synonyms defined as:

Other synonyms of sex include sexual activity, sexual practice.

sex (noun)	sex activity (noun)
activities associated with sexual intercourse	activities associated with sexual intercourse
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)	sex activity (verb)
tell the sex (of young chickens)
to evoke sexual feelings