unfaithful vs sexual activity

unfaithful is not a noun while sexual activity is a noun.

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

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

unfaithful (adjective)	sexual activity (adjective)
not true to duty or obligation or promises
having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend
having the character of, or characteristic of, a traitor
not trustworthy