sex vs go down on

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

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

sex (noun)	go down on (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

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