Truth.

[lunatica]

31. Mr. goes with Mrs.

Ms has only been around since the Feminist movement in the 1960s. It was started by women who wanted to be treated as an independent person and didn’t always want to be identified as a wife, or as single. You know, that second class treatment that your nothing important on your own.

Since we’ve never had a woman President and no one thought we would when titles were given out there is no protocol. Here is a link from Wikipedia addressing the title issue. Evidently Newsweek had a preprinted cover ready if Hillary won in which they referred to her as Madam President.

Scroll down for the reference

https://en.m.wikipedia.org/wiki/Mr._President_(title)#United_States

Evidently titles are what seems to stick. It seems to have happened that way in referring to the First Lady.