sex activity vs improper

sex activity is a noun but improper is not a noun.

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

sex activity (noun)	improper (noun)
activities associated with sexual intercourse

sex activity (adjective)	improper (adjective)
	not suitable or right or appropriate
	not appropriate for a purpose or occasion
	not conforming to legality, moral law, or social convention