When he was twenty-four years old, Leonardo was arrested, along with several young companions, on the charge of sodomy.
No witnesses appeared against them and eventually the charges were dropped. It must be said that often anonymous charges like this were brought against people just for a nuisance. Renaissance Florentines didn't make the distinctions we make about sexuality today and apparently it was common for young men to get into sexual relationships; in fact, the word "Florenzer" was German slang for "homosexual". Leonardo had no relationships with women, never married, had no children, and raised many young protégés, including one nicknamed "Salai" which means "offspring of Satan", sketches of whom are shown above. Salai stole things, broke things, lied, and was generally a, well, devil; if he were a mere student or servant he would have been sacked. It's not hard for me to see how this imp would be attractive to Leonardo (or to me, now that I mention it). He stayed with Leonardo for over twenty years, and appears many times in Leonardo's sketchbooks.
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From Serge Bramly, Leonardo: Discovering the Life of Leonardo Da Vinci (1991):
Among several citations under "Homosexuality" in the index are the following:
"Yet while no definite proof exists of Leonardo's homosexuality, there are plenty of indications, in his drawings as well as in his writings, that he was attracted to males, and, as Freud puts it, 'doubtful whether he ever embraced a woman with love'." p. 119
"There is no record of any woman in his life -- not even a female friendship. On the other hand, he was soon surrounding himself with a constantly renewed court of remarkably beautiful young men." p. 119
There follows six pages on Salai, one of Leonardo's "remarkably beautiful young men", starting at p. 223.
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From Michael White, Leonardo, The First Scientist (2000):
"He [Leonardo] was a homosexual vegetarian born out of wedlock who received very little formal education and was excluded by birthright from almost all professions." p. 7
About the sodomy accusation, he writes:
"It is possible that up to this time he had felt no real guilt about his homosexuality, that it had either been natural to him, or else he accepted it as part of his self-image; after all, there were certainly plenty of role models [in other Florentine painters] for him." p. 72
In describing Leonardo's relationship with Lorenzo de Medici, White writes:
"Lorenzo was heterosexual, later described by Machiavelli as "incredibly given to the pleasures of Venus". Leonardo was homosexual with a very public sodomy trial behind him. However, it is certain the different sexual tastes of the two men would not have been a major factor in the progress of their relationship. After all, Lorenzo actively encouraged at least one other homosexual artist, Michelangelo, giving him a string of commissions and even opening his homes to him, treating him almost as a social equal." p. 83-84
Then, writing about Salai, White says:
"Biographers have skirted the issue of Leonardo's relationship with Salai because it has been a subject too sensitive to investigate candidly. Kenneth Clark, writing in the 1930s, simply refers to the relationship by saying "these facts... inevitably suggest that his relationship with the master was of a kind honored in classical times, and partly tolerated in the renaissance, in spite of the censure of the Church." p. 137
"... Giacomo [Salai's real name] soon became a fixture in his life and was hardly to leave his side for the next three decades. From his actions and the way Leonardo treated him, Salai played the role of son, friend, helper, and quite possibly, lover." p. 137
