Several times the President of the U.S. was not married. In those cases, there was a woman who served as hostess of the White House. Thomas Jefferson was not married during the time he was President. His daughter served as hostess of the White House. Andrew Jackson’s wife died during his time as President. His niece, Emily Donelson, served as hostess of the White House until she too died. Then his daughter-in-law, Sarah Jackson, served as hostess of the White House. James Buchanan was never married. His niece, Harriet Lane, served as hostess of the White House.
Although Jill Biden, Joe Biden’s wife, is referred to as the Second Lady while he was Vice President and now as First Lady while he is President, note that in both cases she has been teaching at a community college at the same time. The real problem here is that the entire idea of First Lady or Second Lady is now out of date. It is no longer necessary that the wife of a President or a Vice President needs to spend all their time being a hostess of the White House.
Many other countries don’t use titles like First Lady for the wife/girl(or boy) friend/whatever of the President/Prime Minister/whatever. It appears in some countries that many people in the country are not even particularly aware of whether the President/Prime Minister/whatever is married or has a girlfriend (or boyfriend). It’s time we gave up the idea of the First/Second Lady/Gentleman entirely. The idea is hopelessly out of date. We should accept that the notion that anyone married to or otherwise involved with the President/Vice President has to spend all their time as hostess/host is just a remnant of how society used to work and should be just forgotten.