well if you think of it alot of kids parents died because of the demon fox but since lee is so extraordinary i personally can't see his parents going out like that. so many people probally doubted lee so it's only naturall for me to assume that they gave up on him as a child because of his chakra issues. and as for him being related to gai its fact that he can't be his father (or mother) but there may be a distant relation...