Emile Griffith is the most widely known that I am aware of. Him just coming out of the closet, welll..
There are stories that I have read that he was angry at Paret before their fateful fight because, at the weigh in and while both were naked to get on the scales, Paret stepped up close behind him and touched him and called him "maricon." I don't know this to be true or false, but I read it someplace years ago.
What I do know is this. A very good friend of mine was Griffith's main sparring partner for svereal fights and my friend said Griffith was openly gay, as well as one hell of a guy and a tremendous fighter.