In what way will we have the first male spouse of a US president? Will it be by virtue of having a straight female president or having a gay or bisexual male president? For this person to count as the first spouse, they need not be legally married to the president as long as they hold themselves out as the president's spouse to the public. For somebody to count as president, they must serve in the office of president, acting president will not count for this role.
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3.00@BrunoParga This would not have counted as James Buchanan did not hold himself out to the public as the husband of William Rufus King. Additionally, it is a note that Harriet Lane would also not count as the first spouse for this question as James Buchanan and her did not hold themselves out to the public as married. This question focuses on the spouse of the president, not the office of the first spouse. Though there hasn't been a distinction between the two in over 100 years, only the spouse of the president would qualify as the first spouse.
@AaronSimansky gotcha, thanks. You've already clarified the answer, but just to restate the question - it is conceivable that there could be a First Gentleman who's the son of a widowed President, for example. As you've defined the question, this situation wouldn't count.
@BrunoParga no, that would not count. I am only counting people who serve as first gentlemen in the modern practice of the position (i.e. the spouse of the president)