Except that w/ sports coaches you are often talking about adults and adolescents. BJJ you are talking about an adult learning from an adult. I don't think it is an issue for a BJJ instructor to date a student, any more than it would be for someone to date their piano teacher, they are both consenting adults. If it becomes a pattern, then it could easily negatively impact the gym's atmosphere, but that is a separate issue than whether one is taking advantage of a supposed position of authority.
In this instance it is an adult and an adolescent