sexual activity vs two-timing

sexual activity is a noun but two-timing is not a noun.

Word	Noun	Adjective	Verb	Adverb
sexual activity	Yes	No	No	No
two-timing	No	Yes	No	No

sexual activity (noun)	two-timing (noun)
activities associated with sexual intercourse

sexual activity (adjective)	two-timing (adjective)
	not faithful to a spouse or lover