sex vs sexual relations

sex

noun
  • Sexual activity, usually sexual intercourse unless preceded by a modifier. 

  • A category into which sexually-reproducing organisms are divided on the basis of their reproductive roles in their species. 

  • Genitalia: a penis or vagina. 

  • Another category, especially of humans and especially based on sexuality or gender roles. 

  • The distinction and relation between these categories, especially in humans; gender. 

  • The members of such a category, taken collectively. 

verb
  • To determine the sex of (an animal). 

  • To have sex with. 

  • To have sex. 

sexual relations

noun
  • The act of sexual intercourse. 

  • Sexual activities between individuals. 

How often have the words sex and sexual relations occurred in a corpus of books? (source: Google Ngram Viewer )