go down on vs sex

go down on is not a noun while sex is a noun.

Word	Noun	Adjective	Verb	Adverb
go down on	No	No	Yes	No
sex	Yes	No	Yes	No

go down on (noun)	sex (noun)
	activities associated with sexual intercourse
	the properties that distinguish organisms on the basis of their reproductive roles
	all of the feelings resulting from the urge to gratify sexual impulses
	either of the two categories (male or female) into which most organisms are divided

go down on (verb)	sex (verb)
provide sexual gratification through oral stimulation	tell the sex (of young chickens)
	to evoke sexual feelings