I've pasted below an excerpt from Buchanan's wikipedia. I think that Andrew Jackson's assertions clearly indicate homosexuality--otherwise, Buchanan would have had to fight with Andrew Jackson in a duel.
For fifteen years in Washington, D.C., prior to his presidency, Buchanan lived with his close friend, Alabama Senator William Rufus King[8]. King became Vice President under Franklin Pierce. He took ill and died shortly after Pierce's inauguration, and four years before Buchanan became President. Buchanan and King's close relation prompted Andrew Jackson to refer to King as "Miss Nancy" and "Aunt Fancy," while Aaron V. Brown spoke of the two as "Buchanan and his wife."[9][10] Further, some of the contemporary press also speculated about Buchanan and King's relationship. Buchanan and King's nieces destroyed their uncles' correspondence, leaving some questions as to what relationship the two men had, but surviving letters illustrate "the affection of a special friendship",[9] and Buchanan wrote of his "communion" with his housemate. Such expression, however, was not unusual amongst men at the time. Though the circumstances surrounding Buchanan and King have led some to speculate that he was America's first homosexual president, there is currently no evidence that King and Buchanan had a sexual relationship.[9]
The only President never to marry, Buchanan turned to Harriet Lane, an orphaned niece whom he had earlier adopted, to act as his First Lady. "I feel that it is not good for man to be alone", he wrote, "and [I] should not be astonished to find myself married to some old maid who can nurse me when I am sick, provide good dinners for me when I am well, and not expect from me any very ardent or romantic affection."[