copulation vs sexual relation

copulation

noun
  • Sexual procreation between a man and a woman or transfer of the sperm from male to female; usually applied to the mating process in nonhuman animals; coitus; coition. 

  • The act of coupling or joining; union; conjunction. 

sexual relation

noun
  • Behaviour of a sexual nature between one or more individuals. 

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