civil marriage vs civil union

civil marriage

noun
  • A marriage performed by a government official instead of by a member of the clergy. 

civil union

noun
  • A legal union similar to marriage, established to allow same-sex couples (and, in some jurisdictions, opposite-sex couples) rights similar to those married couples have. 

How often have the words civil marriage and civil union occurred in a corpus of books? (source: Google Ngram Viewer )