Interesting question? At first I thought of the obvious. The kinship to lady would be gentleman but this name given to husbands of female Governors. So that is out — it would cause confusion.
There are four respectful choices left: First male (or man), First Spouse, First Husband, or First Partner. Right off the bat I scratched off two of the four, not because they would not work in 2016 because they would. I did it because I am someone who (by nature) must think ahead. In the future we will see someone elected who may have someone special (including children) and is unmarried. Furthermore, they may be of the same sex. Lord, help those firsts! Some Americans could barely tolerate the “inexperienced (black) Senator,” or the (gasp) Socialist!
Upon reflection First Male (man) and First Husband will be scratched off when looking toward future elections. What is left is First Partner. I personally hate the sound of that. The word partner conjures up visions of corporations, and the titles like CEO and COO. A President (and whomever) in that visual is more terrifying to me than Zombies or Ghouls! Picture it, America headed for yet another big crash at the hands of M/m CEO and COO. I shudder at just a passing thought of it.
I suppose I prefer First Spouse for long range planning. Unisex enough to cover any marriage or legal union, but does that mean the rules of candidacy must change to ‘married or single (and unspoken for) ?” That could bring forth much dissension. On the other hand when you truly think about it, maybe not. After all, if you say you are skilled and intelligent enough to lead the entire civilized world shouldn’t you, also, be brave enough to sign that little piece of paper?