It's historical revisionism that crosses the line into presentism. Revisionism can be useful: any narrative that is wrong deserves challenging. Most agree history is about interpretation. As much as we claim to search for objective truth, when society changes, so too do our views of past events. Problems arise when we go from reinterpretation to repurposing the past for the present.

Where to draw the line? James W. Loewen's research displays both. Assuming the following anecdote is true (anyone selling books is somewhat suspect):

In the late 1990s, Loewen visited Wheatland, the mansion in Lancaster, Pa., where Buchanan spent his later years.

Loewen said he asked a staffer at Wheatland if Buchanan was gay, and the reply was: “He most definitely was not.”

Loewen said the staffer pointed to a portrait of Ann Coleman, the daughter of a wealthy iron maker, whom Buchanan was engaged to briefly 1819 — shortly before she committed suicide.

However, Loewen scoffed at the staffer’s suggestion that the brief engagement to Coleman proved Buchanan was heterosexual.

It is a case of appropriate revision. The staff were directed to take a neutral stance on Buchanan's sexuality in light of questions new research raised. Here comes the presentism:

“I’m sure that Buchanan was gay,” Loewen said. “There is clear evidence that he was gay. And since I haven’t seen any evidence that he was heterosexual, I don’t believe he was bisexual.”

Wait, what? Insert recitation of Foucault and when the label homosexual came into existence. More important, you can't take a lack of evidence on one side as proof that the other is correct. That's not how history (or science) works. There is sufficient evidence to question Buchanan's sexuality, which is different from declaring him America's first gay president as Loewen wishes to do. We will never know what was going through President Buchanan's head because most of his correspondence was destroyed. What we do know raises questions. Desire for an answer does not give historians licence to supply one.

/u/browsingbill and /u/poopsmearPoogilist are right on the money. Attempting to apply today's identity categories to times before these categories existed is inaccurate and generally bad practice. You might be interested in this rant I wrote on precisely this subject.

But the other thing I want to point out is the prevalence of same-sex 'romantic friendships' in Buchanan's time. These relationships were intense, emotionally charged same-sex relationships which were enabled by the homosociality of many work, religious, educational and other environments of the time. Romantic friendships were accepted and even lauded parts of eighteenth and nineteenth century British and Anglo-North American culture, and they appear frequently in nineteenth century literature.

There is a good deal of scholarly debate about whether these romantic friendships were a socially accepted cover for what we now identify as queer desire, or they were fundamentally different type of relationship.

But romantic friendship is relevant to the Buchanan question because it offers another, potentially 'less gay' reading of his relationships. Granted, according to Robert Watson, Washington gossip in Buchanan's day did treat Buchanan's habits and relationship with William Rufus King with a bit of suspicion, even referring to them as 'Mr. and Mrs. Buchanan' (!). This indicates that Buchanan's contemporaries did see Buchanan's proclivities as somehow outside the realm of hegemonic romantic friendship.

On romantic friendship as a separate phenomenon from queer sexuality, see Carolyn Oulton, Romantic Friendship in Victorian Literature (Aldershot: Ashgate, 2007).

On romantic friendship as a proto-homosexual practice, see Eve Kosofsky Sedgwick, Between Men: English Literature and Male Homosocial Desire (New York: Columbia University Press, 1985).

And for discussion of the gossip surrounding Buchanan during his stint in Washington, see Robert P. Watson, Affairs of State: The Untold History of Presidential Love, Sex, and Scandal, 1789-1900 (Lanham, MD: Rowman & Littlefield, 2012).

I was going to reply to a deleted comment but it is now gone.

So, my related question is: does anybody know of whether or not there is documentation that clearly suggests that there were leaders in other nations, particularly western nations, who were homosexual? I know that there is significant conjecture that Frederick the Great was homosexual or bisexual. How valid is this? Are there other examples?