Sure they wouldn't have considered themselves to be homosexual because the word didn't existed at this time. But if Buchanan only loved men and only found pleasure in sexual intercourse with men, he was homosexual, because his preffered partners belonged to his gender. Many concepts which didn't existed in the past are very well suited to describe phenomena in the past. For example, ancient Greeks didn't used the word economy in the sense we do, but they did have an economy.
Your are saying that it isn't certain that homosexuals in the past might have called themselves homosexuals. But they were gay nonetheless. A homophobic homosexual in modern times might label himself as a heterosexual, he'll still be homosexual.
All I can really do at this point is shrug, because the bolded misses my point. It's not that the
word didn't exist, but it's the
social construction of homosexuality as we know it didn't exist. You can say that, from a modern standpoint, Buchan was gay because he was sexuality attracted to men, and yes to a degree you would be right to label him as a homosexual, but at the same time it ignores his contemporary standpoint in how he and his contemporaries may have viewed such attractions. And you can say that the ancient Greeks had economy, but if anything their considerations of sexuality compared to modern considerations would be a more apt comparison.
BTW, isn't the assertion that Lincoln had romantic/sexual relationship with other men quite uncertain? Buchanan was very likely attracted to men, but Lincoln had a family and a decent relationship with his woman IIRC.
Linocln's relationship with a
specific women was often strained, having what appears to be happier and more close relationships with other men, hence why his sexuality it often brought into question. And just because he had a family...
But to get back on track, with all this considered if a former President came out, or we were given verified proof, then things will get... uncomfortable. You'll likely see a lot of pushback, a lot of skirting around the issue, perhaps even attempts at suppression, but I think that, again, it depends on the President. You may even see the evidence embraced to the homophobic end of blaming the failures of that particular President for their 'gayness'. It would be an issue that would be interesting, but highly dependent.