sexual activity vs improper

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

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

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

sexual 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