sex has 10 distinct synonyms spread across 4 of its 6 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
activities associated with sexual intercourse
“they had sex in the back seat”
either of the two categories (male or female) into which most organisms are divided
“the war between the sexes”
all of the feelings resulting from the urge to gratify sexual impulses
“he wanted a better sex life”
the properties that distinguish organisms on the basis of their reproductive roles
“she didn't want to know the sex of the foetus”
tell the sex (of young chickens)
Synonyms, antonyms and the broader and narrower relations for sex come from Princeton University’s WordNet. Full sources and licences.