Unofficially, the title goes back further. Martha Washington, wife of George, was called the First Lady. So, in a sense, the title is as old as the American Constitution. But, say what you will about Bill Clinton, he ain’t no lady.
He has an eye for the ladies, of course, so calling him the First Lady’s Man would be apt and descriptive, but President Hillary might not like it. When Australia was governed by a woman for the first time, they called Julia Gillard’s partner the First Bloke, but that is not very American. Perhaps First Gentleman would fit the Bill.