sexual activity vs indecent

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

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

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

sexual activity (adjective)	indecent (adjective)
	offensive to good taste especially in sexual matters
	offending against sexual mores in conduct or appearance
	not in keeping with accepted standards of what is right or proper in polite society