What is exciting here is Bill Clinton has so much confidence in Hillary and he will do whatever necessary to give Hillary the most successful term possible for Hillary, they are a team, they do not try to out do the other.

...and it feels awkward to some, but we will adjust quickly with a female president and her spouse.
You pointed out that "FLOTUS" is easy to say and I agree, not to mention we are used to it. I do believe that "Lord" formally pairs to "Lady", and so we would not have to change the FLOTUS acronym at all. "First Lady" is an honorary "title." We don't go on informally referring to her as Lady so-and-so, but "Mrs." and so I don't see the reason that we could not refer to the male spouse of a female president as "First Lord", and then refer to him informally as "Mr. so-and-so", or in the case of a former president, "Sir" or "President" so-and-so.
Just a thought.

I don't know though, "Lady" works well because it's somewhat ambiguous with it's level
of honorifics...ness. "Lord" doesn't work that way really...too "Downton Abbey" ha.
At the same time, yeah..."Man" isn't particularly equal to "Lady" in usage.

Wonder how likely a "bachelor/ette" president is, lol.

First Husband Of The United States. I'd pronounce it as "Foe-tuss."

I was idly thinking that things might get stickier when we elect a married gay person to the Presidency, but FLOTUS and FHOTUS still work very well on second thought.

First Spouse?

Though FGOTUS is not euphonious

The states that have women as governors call their governors' spouses "first gentlemen." The US would be no different.