sexual activity vs obscene

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

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

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

sexual activity (adjective)	obscene (adjective)
	suggestive of or tending to moral looseness
	designed to incite to indecency or lust
	offensive to the mind