sex vs sexual activity

sex and sexual activity both are nouns.

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

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

Other synonyms of sex include sex activity, sexual practice.

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