relationship vs sex partner

relationship

noun
  • A romantic or sexual involvement. 

  • The level or degree of affinity between keys, chords and tones. 

  • The links between the x-values and y-values of ordered pairs of numbers especially coordinates. 

  • Connection or association; the condition of being related. 

  • Kinship; being related by blood or marriage. 

  • A way in which two or more people behave and are involved with each other 

sex partner

noun
  • Someone that one has sex with, especially on an informal or casual basis. 

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