As far as casual wear, a man should not wear shorts that expose more than the knee. Maybe you can go up to an inch higher but anything more looks gay. Admittedly, I did wear those short spandex type when competing in no gi grappling tournaments but that was before they made decent grappling board shorts. The advantage of wearing spandex shorts like Anderson Silva and GSP wears is that your opponent can't grab it and it doesn't get in the way. It gives more freedom of movement. Some of the early board shorts versions would even bind at the knee when you lunged forward for a shot.
