And so Wikipedia spake:
"Little is known about Leonardo's sexuality, as, although he left hundreds of pages of writing, very little of it is personal in nature. He left no letters, poetry or diary that indicate any romantic interest. He never married and it cannot be stated with certainty that he had a sexually intimate relationship with any person, male or female. One of the few references that Leonardo made to sexuality in his notebooks states: "The act of procreation and anything that has any relation to it is so disgusting that human beings would soon die out if there were no pretty faces and sensuous dispositions."[21] This statement has been the subject of various extrapolations and interpretations in attempts to gain a picture of his sexuality."
He was accused of sodomy a couple of times, which was a capital offence at that time, but the accusations were apparently never substantiated.