The accusation that Buchanan was gay comes from a gay historian who interpreted an affectionate letter to a friend as evidence he was gay.
There’s no proof whatsoever, but it has stuck.
They’ve done this with lots of people in history - it’s the old version of demonizing people by calling them ‘racists’.
Walt Whitman comes to mind - because of the affectionate and passionate terms of his time through which he often expressed himself, they decided he was ‘gay’. And they just DECIDE that all sorts of asexual people were ‘gay’.
Some just can’t understand people who happen to think and feel or choose to live differently.