sexual activity vs sex

sexual activity and sex both are nouns.

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

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

Other synonyms of sexual activity include sex activity, sexual practice.

sexual activity (noun)	sex (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

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