Baggy pants or a tucked in shirt increase the using of the zipper, while tight pants require the undoing of the belt. Another nod for genetics and nurturing, the way your daddy does it is usually the way he taught you and is probably how you do it too. While primarily a gay reasoning, undoing the belt promotes the viewing of the member by passers by or other restroom inhabitants. It was mentioned that a smaller proportioned male may be prone to undo his belt, while a well hung person would prefer to use his zipper. Don't shoot the messenger.
Some guys just have a preference due to feel.
More freedom and less constriction lead to the undoing of the belt. Both sides, zipper and belt, argued that their way was easier, there is no conclusive evidence to support either side. The following information is the observations and opinions of people who have been moved to respond to this survey. As a nurse working in a physician's office, I see many male buttocks daily. There are definitely more briefs than boxers.