sexual activity vs illicit

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

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

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

sexual activity (adjective)	illicit (adjective)
	contrary to accepted morality (especially sexual morality) or convention
	contrary to or forbidden by law