Originally posted by Teinosukehttp://uspolitics.about.com/od/gaymarriage/a/Why-Barack-Obama-Was-Called-The-First-Gay-President.htm
In fact it seems that the term was coined for one of these non-marital hostesses!
https://en.wikipedia.org/wiki/First_Lady#History_of_use_in_the_USA
"Harriet Lane, niece of bachelor President James Buchanan, was the first woman to be called first lady while actually serving in that position. The phrase appeared in Frank Leslie's Illustrated Monthly i ...[text shortened]... e Harriet Lane was called first lady, the term was applied retrospectively to her predecessors."